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.

www/firefox/files/patch-dom_media_flac_FlacDecoder.cpp

@@ -17,7 +17,7 @@ Enable FLAC on platforms without ffvpx like powerpc*
 +#elif defined(MOZ_FFMPEG)
 +  RefPtr<PDMFactory> platform = new PDMFactory();
 +  return StaticPrefs::media_flac_enabled() &&
-+         (platform->SupportsMimeType("audio/flac"_ns) != media::DecodeSupport::Unsupported);
++         !platform->SupportsMimeType("audio/flac"_ns).isEmpty();
  #else
    return false;
  #endif