avutil: make some tables visible again

These tables are used for instance by older versions of libavcodec and
need to remain visible until the next SONAME bump.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/libavutil.v

 LIBAVUTIL_$MAJOR {
-        global: av*;
+        global: av*; ff_*_tab;
         local: *;
 };