Add an entry for the recently added av_compare_ts() function.

Originally committed as revision 21713 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-02-07 - r21673 - lavu 50.9.0 - av_compare_ts()
+  Add av_compare_ts().
+
 2010-02-05 - r30513 - lsws 0.10.0 - sws_getCoefficients()
   Add sws_getCoefficients().