gst-ffmpeg: fix build for xtensa

Since commit 6ea49a23 (gst-ffmpeg: Use internal libav instead of external
ffmpeg) gst-ffmpge uses its internal libav. This requires the same fix for
xtensa like ffmpeg got in commit 4ec35e76 (ffmpeg: fix build for xtensa).

Fixes
http://autobuild.buildroot.net/results/08a/08a27891fd5336250f00e611a15ea45505985f67/

.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>