Add entry for the addition of av_match_ext() to the public API.

Originally committed as revision 22749 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-03-31 - r22748 - lavf 52.60.0 - av_match_ext()
+  Make av_match_ext() public.
+
 2010-03-31 - r22736 - lavu 50.14.0 - AVMediaType
   Move AVMediaType enum from libavcodec to libavutil.