x264: Avoid circular dependency with ffmpeg

This won't normally be a problem, but when ffmpeg is updated, it uses x264's
library, which has symbol referenced to the old version of ffmpeg.

This is only a work-around. If users need lavf/lsws support, they can manually
remove it from the x264.mk file.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>