ffmpeg: add swresample and more decoders/parsers to audio-dec variant

[willmo]

Add the AAC decoder + demuxer and the AAC, AC3, FLAC, and MPEG Audio parsers, since these seem obviously important for audio decoding.

Also add the swresample library, which is quite small and seems plausibly useful for audio decoding. For example, shairport-sync requires it for AirPlay 2 support, so this allows changing shairport-sync's dependency from libffmpeg-full to libffmpeg-audio-dec, which makes for a significant reduction in image size.

I verified that shairport-sync AirPlay 2 still works correctly with its dependency changed to libffmpeg-audio-dec. (I'll submit a separate PR for that if this is accepted.)

Maintainer: @neheb @BKPepe @thess Compile/run tested: ramips, Cudy TR1200 v1, OpenWrt 24.10-SNAPSHOT, r28242-1eff737906

CC #22993 @mikebrady @bklang