Call Update before giving away focus on dismiss menu in wxUniv

Without immediate refreshing menu is appearing as inactive.

Closes https://github.com/wxWidgets/wxWidgets/pull/2479

src/univ/menu.cpp

@@ -2563,6 +2563,7 @@ void wxMenuBar::OnDismiss()
         m_current = -1;
 
         RefreshItem(current);
+        Update();
     }
 
     GiveAwayFocus();