Fix usage with C++ modules

[fedapo]

Fix a compiler error when including Boost.Fusion headers from a C++ module header unit.
Note that one error was not by direct inclusion of Boost.Fusion headers, but through Boost.Spirit.X3.
The errors are the following:

FAILED: [code=1] CMakeFiles/prova_modules.dir/my_module.ixx.o CMakeFiles/prova_modules.dir/mio.modulo.gcm
/usr/bin/c++  -isystem /vcpkg/installed/x64-linux-release/include -O3 -DNDEBUG -std=gnu++26 -Wall -Wextra -MD -MT CMakeFiles/prova_modules.dir/my_module.ixx.o -MF CMakeFiles/prova_modules.dir/my_module.ixx.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/prova_modules.dir/my_module.ixx.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/prova_modules.dir/my_module.ixx.o -c <...>/my_module.ixx
In file included from /vcpkg/installed/x64-linux-release/include/boost/fusion/container/vector/detail/at_impl.hpp:12,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/container/vector/vector.hpp:29,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/tuple/tuple.hpp:24,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/tuple.hpp:11,
                 from <...>/my_module.ixx:36:
/vcpkg/installed/x64-linux-release/include/boost/fusion/container/vector/detail/value_at_impl.hpp:50:20: error: ‘template<class Sequence, class N> struct boost::fusion::extension::value_at_impl<boost::fusion::vector_tag>::apply’ exposes TU-local entity ‘template<long unsigned int N, class U> boost::mpl::identity<Key> boost::fusion::vector_detail::value_at_impl(const volatile store<N, U>*)’
   50 |             struct apply : BOOST_FUSION_DECLTYPE_N3031((
      |                    ^~~~~
/vcpkg/installed/x64-linux-release/include/boost/fusion/container/vector/detail/value_at_impl.hpp:38:26: note: ‘template<long unsigned int N, class U> boost::mpl::identity<Key> boost::fusion::vector_detail::value_at_impl(const volatile store<N, U>*)’ declared with internal linkage
   38 |         mpl::identity<U> value_at_impl(store<N, U> const volatile*);
      |                          ^~~~~~~~~~~~~
ninja: build stopped: subcommand failed.

FAILED: [code=1] CMakeFiles/prova_modules.dir/my_module.ixx.o CMakeFiles/prova_modules.dir/mio.modulo.gcm
/usr/bin/c++  -isystem /vcpkg/installed/x64-linux-release/include -O3 -DNDEBUG -std=gnu++26 -Wall -Wextra -MD -MT CMakeFiles/prova_modules.dir/my_module.ixx.o -MF CMakeFiles/prova_modules.dir/my_module.ixx.o.d -fmodules-ts -fmodule-mapper=CMakeFiles/prova_modules.dir/my_module.ixx.o.modmap -MD -fdeps-format=p1689r5 -x c++ -o CMakeFiles/prova_modules.dir/my_module.ixx.o -c <...>/my_module.ixx
In file included from /vcpkg/installed/x64-linux-release/include/boost/fusion/iterator/segmented_iterator.hpp:14,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/sequence/intrinsic/detail/segmented_begin.hpp:12,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/sequence/intrinsic/begin.hpp:18,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/container/deque/deque.hpp:36,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/container/deque.hpp:13,
                 from /vcpkg/installed/x64-linux-release/include/boost/fusion/include/deque.hpp:11,
                 from /vcpkg/installed/x64-linux-release/include/boost/spirit/home/x3/support/traits/container_traits.hpp:14,
                 from /vcpkg/installed/x64-linux-release/include/boost/spirit/home/x3/auxiliary/any_parser.hpp:15,
                 from /vcpkg/installed/x64-linux-release/include/boost/spirit/home/x3/auxiliary.hpp:11,
                 from /vcpkg/installed/x64-linux-release/include/boost/spirit/home/x3.hpp:62,
                 from <...>/my_module.ixx:35:
/vcpkg/installed/x64-linux-release/include/boost/fusion/container/list/cons.hpp:83:9: error: ‘template<class Car, class Cdr> template<class Sequence> boost::fusion::cons<Car, Cdr>::cons(const Sequence&, typename boost::enable_if<boost::mpl::and_<boost::fusion::traits::is_sequence<Sequence>, boost::mpl::not_<boost::is_base_of<boost::fusion::cons<Car, Cdr>, Sequence> >, boost::mpl::not_<boost::is_convertible<Sequence, Car> > >, boost::fusion::detail::enabler_>::type)’ exposes TU-local entity ‘boost::fusion::detail::enabler’
   83 |         cons(
      |         ^~~~
In file included from /vcpkg/installed/x64-linux-release/include/boost/fusion/container/deque/deque.hpp:25:
/vcpkg/installed/x64-linux-release/include/boost/fusion/support/detail/enabler.hpp:17:33: note: ‘boost::fusion::detail::enabler’ declared with internal linkage
   17 | BOOST_STATIC_CONSTEXPR enabler_ enabler = {};
      |                                 ^~~~~~~
ninja: build stopped: subcommand failed.