package/gstreamer1/gst1-libav: not available on nios2

ffmpeg is not available on nios2, this limitation was not reverse-
propagated to this package.

Fixes
http://autobuild.buildroot.net/results/267/2674395c1d804df68e7e4bc9bba1d6b7fc6d2dce/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>