Add explanatory comment to '#define FLOAT float'.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@12254 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/faanidct.c

@@ -20,6 +20,7 @@
  */
 #include "mpegvideo.h"
 
+/* To allow switching to double. */
 #define FLOAT float
 
 #define B0 1.0000000000000000000000