Support ffmpeg 9

alsa-project/alsa-plugins issue #64 was opened from 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

Issue URL     : https://github.com/alsa-project/alsa-plugins/issues/64
Repository URL: https://github.com/alsa-project/alsa-plugins