Add APIchanges entry for av_get_codec_tag_string().

Originally committed as revision 23424 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-06-02 - r23421 - lavc 52.73.0 - av_get_codec_tag_string
+  Add av_get_codec_tag_string().
+
 2010-06-01 - r31301 - lsws 0.11.0 - convertPalette API
   Add sws_convertPalette8ToPacked32 and sws_convertPalette8ToPacked24