Hide demuxers', muxers' and protocols' objects via the ld version script.

This reduces the symbols exported by libavformat from 699 to 451.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/libavformat.v

 LIBAVFORMAT_$MAJOR {
         global: *;
+        local:
+                ff_*_demuxer;
+                ff_*_muxer;
+                ff_*_protocol;
 };