Removed a lot of occurences of windows.h (and added necessary other
includes).

diff --git a/misc/callback.c b/misc/callback.c
index fe489ac..83d77e7 100644
--- a/misc/callback.c
+++ b/misc/callback.c
@@ -5,12 +5,12 @@
  */
 
 #include <assert.h>
-#include "debug.h"
-#include "windows.h"
+#include "winuser.h"
 #include "callback.h"
 #include "task.h"
 #include "syslevel.h"
 #include "queue.h"
+#include "debug.h"
 
 
 /**********************************************************************