obs-ffmpeg: Include channel_layout.h

avcodec.h stopped including channel_layout.h per FFmpeg commit
1be3d8a0cb77 [1]. avformat.h stopped including avcodec.h per FFmpeg
commit e67e02d15672 [2]. As a result, we need to explicitly include
avutil/channel_layout.h when needed. Fixes compilation error against
FFmpeg later than the two mentioned commits.

[1]: https://github.com/FFmpeg/FFmpeg/commit/1be3d8a0cb77f8d34c1f39b47bf5328fe10c82d7
[2]: https://github.com/FFmpeg/FFmpeg/commit/e67e02d15672a87da1b0566e197a1e19dc7e1e33