Add an APIchanges entry after the M_PHI addition.

Originally committed as revision 24442 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2010-07-23 - r24439 - lavu 50.23.0 - mathematics.h
+  Add the M_PHI constant definition.
+
 2010-07-22 - r24424 - lavfi 1.26.0 - media format generalization
   Add a type field to AVFilterLink.