audio/flac: add USES=compiler:c11

This is to fix audio/audacity:
/usr/local/bin/ld: ondemand/audacity-ODDecodeFlacTask.o:(.data.rel.ro._ZTV10ODFLACFile[_ZTV10ODFLACFile]+0x148): undefined reference to `FLAC::Decoder::File::init(std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/local/bin/ld: ondemand/audacity-ODDecodeFlacTask.o:(.data.rel.ro._ZTV10ODFLACFile[_ZTV10ODFLACFile]+0x160): undefined reference to `FLAC::Decoder::File::init_ogg(std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/local/bin/ld: import/audacity-ImportFLAC.o:(.data.rel.ro._ZTV10MyFLACFile[_ZTV10MyFLACFile]+0x148): undefined reference to `FLAC::Decoder::File::init(std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/local/bin/ld: import/audacity-ImportFLAC.o:(.data.rel.ro._ZTV10MyFLACFile[_ZTV10MyFLACFile]+0x160): undefined reference to `FLAC::Decoder::File::init_ogg(std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'

Approved by: mentors (implicite approval)