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.