Removed #include of wingdi.h and windef.h from winuser.h (and resolved
the resulting compilation failures).

diff --git a/windows/msgbox.c b/windows/msgbox.c
index 4750b20..62495c2 100644
--- a/windows/msgbox.c
+++ b/windows/msgbox.c
@@ -6,6 +6,8 @@
 
 #include <string.h>
 
+#include "windef.h"
+#include "wingdi.h"
 #include "wine/winbase16.h"
 #include "wine/winuser16.h"
 #include "dlgs.h"