Add APIchanges entry for av_codec_get_tag2().

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2012-10-22
 
 API changes, most recent first:
 
+2013-01-17 - a1a707f - lavf 54.61.100
+  Add av_codec_get_tag2().
+
 2012-12-20 - xxxxxxx - lavfi 3.28.100 - avfilter.h
   Add AVFilterLink.channels, avfilter_link_get_channels()
   and avfilter_ref_get_channels().