Hide the now-prefixed decoders, encoders, parsers, bsf, hwaccel objects.

This significantly reduces the size of the symbol table in the generated ELF
shared object (as well as the other linked tables).
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/libavcodec.v

 LIBAVCODEC_$MAJOR {
         global: *;
+        local:
+                ff_*_bsf;
+                ff_*_decoder;
+                ff_*_encoder;
+                ff_*_hwaccel;
+                ff_*_parser;
 };