Make sure that all MDI children have the WS_EX_MDICHILD style set.

diff --git a/windows/mdi.c b/windows/mdi.c
index e3674e8..3547b44 100644
--- a/windows/mdi.c
+++ b/windows/mdi.c
@@ -372,6 +372,9 @@
     {
 	WND* wnd = WIN_FindWndPtr( hwnd );
 
+	/* All MDI child windows have the WS_EX_MDICHILD style */
+	wnd->dwExStyle |= WS_EX_MDICHILD;
+
 	MDI_MenuModifyItem(w ,hwnd); 
 	if( wnd->dwStyle & WS_MINIMIZE && ci->hwndActiveChild )
 	    ShowWindow( hwnd, SW_SHOWMINNOACTIVE );