[SDK:CRT] Implement vswprintf()

SDK/INC/CRT/wchar.h

@@ -22,6 +22,7 @@ Abstract:
 extern "C" {
 #endif
 
+#include <stdarg.h>
 #include <string.h>
 
 size_t wcslen(const wchar_t *str);
@@ -35,6 +36,8 @@ wchar_t *wmemset(wchar_t *dest, wchar_t c, size_t count);
 wchar_t *wmemcpy(wchar_t *dest, const wchar_t *src, size_t count);
 wchar_t *wmemmove(wchar_t *dest, const wchar_t *src, size_t count);
 
+int vswprintf(wchar_t *wcs, size_t maxlen, const wchar_t *format, va_list args);
+
 #ifdef __cplusplus
 }
 #endif