core: use NM_SHUTDOWN_TIMEOUT_5000_MSEC define in _ppp_manager_stop()

The define makes it clearer that there is an important relationship
between the timeout for the async operation, and the wrapup time when
NetworkManager is quitting. Well, not for the time being. But in the future,
when we rework the quitting of NetworkManager.

src/core/ppp/nm-ppp-manager.c

@@ -1246,7 +1246,7 @@ _ppp_manager_stop(NMPPPManager            *self,
                               SIGTERM,
                               LOGD_PPP,
                               "pppd",
-                              5000,
+                              NM_SHUTDOWN_TIMEOUT_5000_MSEC,
                               _stop_child_cb,
                               handle);