databases/soci: fix build with GCC-based architectures

Add USES=compiler:c++11-lang to fix linking issue against libmysqlclient.so:
/usr/local/lib/mysql/libmysqlclient.so: undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_create(unsigned long&, unsigned long)@GLIBCXX_3.4.21'

Approved by: mentors (implicit approval)