Some more recursive include fixes/optimizations.

diff --git a/windows/mdi.c b/windows/mdi.c
index 250255a..5204be0 100644
--- a/windows/mdi.c
+++ b/windows/mdi.c
@@ -15,7 +15,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>
-#include "windows.h"
+#include "winuser.h"
 #include "win.h"
 #include "heap.h"
 #include "nonclient.h"