[build] Define __WINT_TYPE__ if necessary

sparse does not define __WCHAR_TYPE__ or __WINT_TYPE__.  We already
define __WCHAR_TYPE__ if the compiler does not do so; do the same for
__WINT_TYPE__.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/stddef.h

 #ifndef __WCHAR_TYPE__
 #define __WCHAR_TYPE__ long int
 #endif
+#ifndef __WINT_TYPE__
+#define __WINT_TYPE__ long int
+#endif
 typedef __WCHAR_TYPE__ wchar_t;
 typedef __WINT_TYPE__ wint_t;