Add explanatory comment to '#define FLOAT float'.

Originally committed as revision 12254 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/faanidct.c

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