Add missing float.h include to snprintf.c.

On windows _isnan() (which isnan() is redirected to in port/win32.h)
is declared in float.h, not math.h.

Per buildfarm animal currawong.

Backpatch to all supported branches.

src/port/snprintf.c

@@ -33,6 +33,9 @@
 #include "c.h"
 
 #include <ctype.h>
+#ifdef _MSC_VER
+#include <float.h>				/* for _isnan */
+#endif
 #include <limits.h>
 #include <math.h>
 #ifndef WIN32