libav: library packages shouldn't rdepend on libav package

Otherwise the libav executables (avplay, ffmpeg etc.) get dragged
into the image, along with all their dependencies (e.g. SDL).

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Acked-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>