Moved all Win16 definitions out of the standard Windows headers.

diff --git a/include/message.h b/include/message.h
index a9d2510..29decbc 100644
--- a/include/message.h
+++ b/include/message.h
@@ -8,6 +8,7 @@
 #define __WINE_MESSAGE_H
 
 #include "windef.h"
+#include "wine/windef16.h"
 
 struct tagMSG;