www/firefox: fix build on powerpc64

In file included from Unified_cpp_common_audio_gn0.cpp:101:
/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/common_audio/wav_file.cc:93:2: error: "Need to convert samples to big-endian when reading from WAV file"
^
/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/common_audio/wav_file.cc:133:2: error: "Need to convert samples to big-endian when reading from WAV file"
^
/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/common_audio/wav_file.cc:217:2: error: "Need to convert samples to little-endian when writing to WAV file"
^
/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/common_audio/wav_file.cc:247:2: error: "Need to convert samples to little-endian when writing to WAV file"
^
In file included from Unified_cpp_common_audio_gn0.cpp:110:
/wrkdirs/usr/ports/www/firefox/work/firefox-97.0/third_party/libwebrtc/common_audio/wav_header.cc:30:2: error: "Code not working properly for big endian platforms."
^