Fixed header dependencies to be fully compatible with the Windows
headers (with help from Dimitrie O. Paun).

diff --git a/windows/class.c b/windows/class.c
index 3ae47f4..c4e7400 100644
--- a/windows/class.c
+++ b/windows/class.c
@@ -29,12 +29,14 @@
 #include "config.h"
 #include "wine/port.h"
 
+#include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
 
 #include "wine/winbase16.h"
 #include "winerror.h"
 #include "windef.h"
+#include "winbase.h"
 #include "wingdi.h"
 #include "wine/winuser16.h"
 #include "wownt32.h"