revert previous not-ready-for-primetime checkin.

winsup/cygwin/winsup.h

@@ -217,6 +217,8 @@ void events_terminate (void);
 
 void __stdcall close_all_files ();
 
+/* Invisible window initialization/termination. */
+HWND __stdcall gethwnd (void);
 /* Check if running in a visible window station. */
 extern bool has_visible_window_station (void);