Support ffmpeg 9

[lfos]

Ffmpeg 9 has been released today [1].

Compilation with ffmpeg 9 currently fails:

pcm_a52.c: In function '_snd_pcm_a52_open':
pcm_a52.c:1159:28: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1159 |                 rec->codec = avcodec_find_encoder_by_name(avcodec);
      |                            ^
pcm_a52.c:1161:28: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1161 |                 rec->codec = avcodec_find_encoder_by_name("ac3_fixed");
      |                            ^
pcm_a52.c:1163:36: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1163 |                         rec->codec = avcodec_find_encoder_by_name("ac3");
      |                                    ^
pcm_a52.c:1166:28: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1166 |                 rec->codec = avcodec_find_encoder(AV_CODEC_ID_AC3);
      |                            ^
pcm_a52.c:1205:36: error: 'AVCodec' has no member named 'sample_fmts'
 1205 |         rec->av_format = rec->codec->sample_fmts[0];
      |                                    ^~

[1] https://ffmpeg.org/download.html

[lfos]

FYI, we're currently using this patch for the Arch Linux package: https://gitlab.archlinux.org/archlinux/packaging/packages/alsa-plugins/-/blob/7f250af93b76e9b3d552af509beb8ea1356114d1/ffmpeg9.patch (may need to be updated to work with older ffmpeg versions).