MFC r300998:

Add missing va_list to <wchar.h>.

  It looks like va_list should always be defined when XSI is enabled. It
  moved over to the POSIX base in the 2008 edition.

include/wchar.h

@@ -76,6 +76,13 @@ typedef	__size_t	size_t;
 #define	_SIZE_T_DECLARED
 #endif
 
+#if __POSIX_VISIBLE >= 200809 || __XSI_VISIBLE
+#ifndef _VA_LIST_DECLARED
+typedef	__va_list	va_list;
+#define	_VA_LIST_DECLARED
+#endif
+#endif
+
 #ifndef	__cplusplus
 #ifndef _WCHAR_T_DECLARED
 typedef	___wchar_t	wchar_t;