[build] Default to short wchar_t in stddef.h

sparse does not understand -fshort-wchar.  Default to using uint16_t
as a wchar_t if not explicitly specified by the compiler, to avoid
large numbers of spurious warnings from sparse.

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

src/include/stddef.h

 
 /* __WCHAR_TYPE__ is defined by gcc and will change if -fshort-wchar is used */
 #ifndef __WCHAR_TYPE__
-#define __WCHAR_TYPE__ long int
+#define __WCHAR_TYPE__ uint16_t
 #endif
 #ifndef __WINT_TYPE__
-#define __WINT_TYPE__ long int
+#define __WINT_TYPE__ int
 #endif
 typedef __WCHAR_TYPE__ wchar_t;
 typedef __WINT_TYPE__ wint_t;