www/firefox-esr: fix build on powerpc64*

In file included from Unified_cpp_dom_media_flac0.cpp:2:
/wrkdirs/usr/ports/www/firefox-esr/work/firefox-115.4.0/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);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/wrkdirs/usr/ports/www/firefox-esr/work/firefox-115.4.0/dom/media/flac/FlacDecoder.cpp:20:44: error: invalid operands to binary expression ('StripAtomic<mozilla::RelaxedAtomicBool>' (aka 'bool') and 'media::DecodeSupportSet' (aka 'EnumSet<mozilla::media::DecodeSupport, unsigned long>'))
  return StaticPrefs::media_flac_enabled() &&
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^
2 errors generated.

www/firefox-esr/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);
++         !platform->SupportsMimeType("audio/flac"_ns).isEmpty();
  #else
    return false;
  #endif