Add float.h include to int8.c, for isnan().

port.h redirects isnan() to _isnan() on windows, which in turn is
provided by float.h rather than math.h. Therefore include the latter
as well.

Per buildfarm.

src/backend/utils/adt/int8.c

@@ -14,6 +14,7 @@
 #include "postgres.h"
 
 #include <ctype.h>
+#include <float.h> /* for _isnan */
 #include <limits.h>
 #include <math.h>