Move HDWP to winuser.h.
Convert HWINSTA to STRICT.

diff --git a/windows/user.c b/windows/user.c
index 9f603a6..edfa779 100644
--- a/windows/user.c
+++ b/windows/user.c
@@ -537,7 +537,7 @@
 	FIXME_(win32)("(%s,0x%08lx,0x%08lx,%p),stub!\n",debugstr_w(winstation),
 		res1,desiredaccess,lpsa
 	);
-	return 0xdeadcafe;
+	return (HWINSTA)0xdeadcafe;
 }
 
 /***********************************************************************