avcodec/amr: replace #define by typedef

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

libavcodec/amr.h

@@ -28,9 +28,9 @@
 #include "avcodec.h"
 
 #ifdef AMR_USE_16BIT_TABLES
-#define R_TABLE_TYPE uint16_t
+typedef uint16_t R_TABLE_TYPE;
 #else
-#define R_TABLE_TYPE uint8_t
+typedef uint8_t R_TABLE_TYPE;
 #endif
 
 /**