include: Include a missing header

[Andrew Talbot]

This is needed for the various variadic functions, such as RtlFormatMessage().

-- Andy.
---
Changelog:
    include: Include a missing header.

diff -urN a/include/winternl.h b/include/winternl.h
--- a/include/winternl.h	2007-03-26 14:13:56.000000000 +0100
+++ b/include/winternl.h	2007-04-21 16:58:24.000000000 +0100
@@ -21,6 +21,7 @@
 #ifndef __WINE_WINTERNL_H
 #define __WINE_WINTERNL_H
 
+#include <stdarg.h>
 #include <windef.h>
 
 #ifdef __cplusplus