Add APIchanges entry after av_strerror() addition.

Originally committed as revision 22685 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-03-25 - r22684 - lavu 50.13.0 - av_strerror()
+  Implement av_strerror().
+
 2010-03-23 - r22649 - lavc 52.60.0 - av_dct_init()
   Support DCT-I and DST-I