Add APIchanges entry for the av_metadata_copy() addition.

Originally committed as revision 26361 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2011-01-14 - r26330 - lavf 52.93.0 - av_metadata_copy()
+  Add av_metadata_copy() in avformat.h.
+
 2011-01-07 - r26262 - lavc 52.107.0 - deprecate reordered_opaque
   Deprecate reordered_opaque in favor of pkt_pts/dts.