avcodec: Add missing FF_API define for libavcodec pix fmt API

avcodec: Add missing FF_API define for libavcodec pix fmt API

avcodec_find_best_pix_fmt2 has been deprecated and replaced by
avcodec_find_best_pix_fmt_of_2 in 2a54ae9df8cbc1717b3929222ac75f384e2ff240.
avcodec_find_best_pix_fmt_of_2 and avcodec_get_pix_fmt_loss meanwhile
were deprecated in 617e866e25b72fa5d9f9d6bbcbd7e4bd69e63a54 when these
functions were de facto moved to libavutil; this has been mentioned in
APIchanges in f7a1c5e4d2294a8970ede7f6deb2fe0a64e202a5. Yet the
attribute_deprecated was never set for the latter two functions and they
were not wrapped in an FF_API define. This commit does this.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>