Include the same headers as on Windows.

diff --git a/include/comcat.h b/include/comcat.h
index 05e1374..2d8563d 100644
--- a/include/comcat.h
+++ b/include/comcat.h
@@ -4,6 +4,13 @@
 #include "rpc.h"
 #include "rpcndr.h"
 
+#ifndef COM_NO_WINDOWS_H
+#include "windows.h"
+#include "ole2.h"
+#endif
+
+#include "unknwn.h"
+
 typedef GUID CATID;
 typedef REFGUID REFCATID;