avcodec/Makefile: add libutvideo.h to SKIPHEADERS

libutvideo.h is not a C header and thus fails building as a C file

Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/audio_fifo: split into a separate doxy module

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>