Workaround for broken C++17 stuctured binding in old compilers

[denzor200]

This PR fixes compile-time errors like this:

/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../../include/c++/5.5.0/tuple:807:33: error: implicit instantiation of undefined template 'std::tuple_size<my_struct>'
    : integral_constant<size_t, tuple_size<_Tp>::value> { };
                                ^
./boost/pfr/detail/core17_generated.hpp:57:9: note: in instantiation of template class 'std::tuple_size<const my_struct>' requested here
  auto& [a,b,c] = val; // ====================> Boost.PFR: User-provided type is not a SimpleAggregate.

[coveralls]

Pull Request Test Coverage Report for Build 3876622619

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 3862777246:	0.0%
Covered Lines:	372
Relevant Lines:	372

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💛 - Coveralls

[denzor200]

This snippet shows that the fix works: https://godbolt.org/z/6YKh335q3