Add an entry for the av_bmg_next() function addition.

Originally committed as revision 20809 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2009-12-12 - r20808 - lavu 50.6.0 - av_bmg_next()
+  Introduce the av_bmg_next() function.
+
 2009-12-05 - r20734 - lavfi 1.12.0 - avfilter_draw_slice()
   Add a slice_dir parameter to avfilter_draw_slice().