ffmpeg: Fix build on ppc32

Fixes
| powerpc-yoe-linux/11.2.0/ld: libavformat/libavformat.so: undefined reference to `__atomic_fetch_sub_8'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>