This fixes a compile error with recent upstream FFmpeg.
The AV_CODEC_CAP_AUTO_THREADS was deprecated and renamed to
AV_CODEC_CAP_OTHER_THREADS in FFmpeg upstream commit
7d09579190de (lavc 58.132.100).
The AV_CODEC_CAP_AUTO_THREADS was finally removed in FFmpeg upstream
commit 10c9a0874cb3 (lavc 59.63.100).
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3951>