lavfi: stop exporting internal functions

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

libavfilter/libavfilter.v

 LIBAVFILTER_$MAJOR {
         global: avfilter_*; av_*;
-            ff_default_query_formats;
         local: *;
 };