Do not include wctype.h in wchar.h with _GNU_SOURCE

_GNU_SOURCE generally enables all features, but in this case the POSIX
requirement to #include <wctype.h> for these is preferred.

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

newlib/libc/include/wchar.h

@@ -33,7 +33,7 @@ typedef __gnuc_va_list va_list;
 #endif
 #endif /* __POSIX_VISIBLE >= 200809 || __XSI_VISIBLE */
 
-#if __XSI_VISIBLE /* && __XSI_VISIBLE < 800 */
+#if __XSI_VISIBLE /* && __XSI_VISIBLE < 800 */ && !__GNU_VISIBLE
 #include <wctype.h>
 #endif