[FFmpeg-soc] build system changes

[Diego Biurrun]

I just changed the way #includes are handled in FFmpeg.  You need to
update your stuff to match this.  #includes residing in other
directories now require the full path from the root of the source tree,
i.e.:

#include "avcodec.h"

-->

#include "libavcodec/avcodec.h"

and similar.  Happy committing.

Diego