www/firefox: fix build on powerpc64*
In file included from Unified_cpp_dom_media_flac0.cpp:2:
/wrkdirs/usr/ports/www/firefox/work/firefox-105.0/dom/media/flac/FlacDecoder.cpp:21:10: error: value of type 'media::DecodeSupportSet' (aka 'EnumSet<mozilla::media::DecodeSupport, unsigned long>') is not contextually convertible to 'bool'
platform->SupportsMimeType("audio/flac"_ns);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~