avcodec/libxavs: disable global header code

This code only segfaults and fixing the segfault, the resulting
files are unplayable, so disable to avoid the segfault.

Better solution is welcome

See: [FFmpeg-devel] [PATCH] avcodec/libxavs: remove global header code
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>