- Adjust disable-log-out gschema key usage for org.mate.lockdown change

By commit d16d491d2fbe94e3c0711c9b5282734e0f26d33f to the mate-panel; disable-log-out key moved to mate-lockdown gschema thus gnome-main-menu needs to know where this key is. This commit modifies main-menu/scr/main-menu-ui to reflect this change. If it is used with monsta's https://github.com/GNOME/gnome-main-menu/pull/2 request we can have a working gnome-main-menu with Mate 1.9.x