x264: disable avxsynth support

Fixes:
http://autobuild.buildroot.net/results/0f8/0f8b1167ba23a9a2cd9bd6b6e1baa0470523d47d/
http://autobuild.buildroot.net/results/b65/b651f488fbd6f2985e1a656814098db6c60040e5/



We don't have an avxsynth package in Buildroot, and x264 uses dlopen to
access it so it breaks on static only toolchains.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>