avcodec/faanidct: replace #define by typedef

See e.g https://stackoverflow.com/questions/1666353/are-typedef-and-define-the-same-in-c
for rationale.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/faanidct.c

@@ -22,7 +22,7 @@
 #include "libavutil/common.h"
 
 /* To allow switching to double. */
-#define FLOAT float
+typedef float FLOAT;
 
 #define B0 1.0000000000000000000000
 #define B1 1.3870398453221474618216 // cos(pi*1/16)sqrt(2)