www/firefox: fix build on powerpc64*
In file included from Unified_cpp_dom_media_flac0.cpp:2:
/wrkdirs/usr/ports/www/firefox/work/firefox-119.0/dom/media/flac/FlacDecoder.cpp:21:80: error: no member named 'Unsupported' in 'mozilla::media::DecodeSupport'
(platform->SupportsMimeType("audio/flac"_ns) != media::DecodeSupport::Unsupported);
~~~~~~~~~~~~~~~~~~~~~~^1 error generated.