gst1-libav: fix build for xtensa

gst1-libav needs the same fix for xtensa like ffmpeg got in commit
4ec35e76 (ffmpeg: fix build for xtensa).

Fixes:
http://autobuild.buildroot.net/results/33f/33fcbb79809719f68f3ee2cdb0fcacad82b9e0eb/



[Peter: pass the correct flags, not the ones from ffmpeg]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>