avcodec/libavcodec.v: Tighten export whitelist

Currently every symbol (with external linkage) that starts with "av" is
exported. Yet libaom-av1 has lots of functions that are not meant to be
exported and start with "av1_" (I counted 1236); and libvpx has
average_split_mvs. These functions are exported if one links these
libraries statically into a shared libavcodec.so.

Solve this by tightening the whitelist to "av_", "avcodec_", "avpriv_"
and (as a special-case) "avsubtitle_free".
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/libavcodec.v

 LIBAVCODEC_MAJOR {
     global:
-        av*;
+        av_*;
+        avcodec_*;
+        avpriv_*;
+        avsubtitle_free;
     local:
         *;
 };