[GCC] link error: multiple definitions

[hrobeers]

Fix for the following link errors in GCC:

expected/include/boost/expected/expected.hpp:95: error: multiple definition of boost::rethrow(boost::exception_ptr)' expected/include/boost/expected/expected.hpp:99: error: multiple definition ofboost::make_error_from_current_exception(boost::exception_ptr)' expected/include/boost/expected/expected.hpp:113: error: multiple definition of std::rethrow(std::__exception_ptr::exception_ptr)' expected/include/boost/expected/expected.hpp:117: error: multiple definition ofstd::make_error_from_current_exception(std::__exception_ptr::exception_ptr)'

[viboes]

Thanks for this PR.