gst-ffmpeg: disable for MIPS R6

This package bundles a version of libav which doesn't have support for
MIPS R6.

Fixes:

  http://autobuild.buildroot.org/results/e54/e54f0f9aa8fbc1760379ce64d17422810864b56b/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>