avutil: Add deprecation ifdefs around obsolete intfloat code

This long-deprecated code should disappear at the next version bump.

libavutil/intfloat_readwrite.c

@@ -28,7 +28,9 @@
 #include <stdint.h>
 #include "mathematics.h"
 #include "intfloat_readwrite.h"
+#include "version.h"
 
+#if FF_API_INTFLOAT
 double av_int2dbl(int64_t v){
     if((uint64_t)v+v > 0xFFEULL<<52)
         return NAN;
@@ -95,3 +97,4 @@ AVExtFloat av_dbl2ext(double d){
         ext.exponent[0] |= 0x80;
     return ext;
 }
+#endif /* FF_API_INTFLOAT */