intfloat_readwrite: include "mathematics.h" for fallback macros

This allows this file to build on systems lacking NAN or INFINITY
in math.h.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/intfloat_readwrite.c

@@ -26,7 +26,7 @@
  */
 
 #include <stdint.h>
-#include <math.h>
+#include "mathematics.h"
 #include "intfloat_readwrite.h"
 
 double av_int2dbl(int64_t v){