lavu: stop exporting internal functions

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/libavutil.v

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