libavutil.v: add avpriv prefix

This should fix the fate failure
http://fate.ffmpeg.org/log.cgi?time=20120609002213&log=compile&slot=x86_64-archlinux-gcc-enablesharedSigned-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/libavutil.v

 LIBAVUTIL_$MAJOR {
-        global: av_*; ff_*; avutil_*;
+        global: av_*; ff_*; avutil_*; avpriv_*;
         local: *;
 };