Moved all Win16 definitions out of the standard Windows headers.

diff --git a/include/thread.h b/include/thread.h
index b0c6aeb..72a3ea8 100644
--- a/include/thread.h
+++ b/include/thread.h
@@ -10,6 +10,7 @@
 #include "config.h"
 
 #include "ntdef.h" /* UNICODE_STRING */
+#include "wine/windef16.h"
 
 struct _PDB;
 struct __EXCEPTION_FRAME;