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>