libav: guard some recently dropped APIs
The FF_API_AYUV_CODECID api was dropped in upstream:
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/9ee59b63f5ea
The FF_API_OLD_CHANNEL_LAYOUT api was dropped in upstream:
https://git.ffmpeg.org/gitweb/ffmpeg.git/commit/65ddc7498824