Add missing return in shutdown

Co-Authored-By: Jason <armooo@armooo.net>
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/app.go

@@ -775,6 +775,8 @@ func (h *Headscale) Serve() error {
 
 				// And we're done:
 				cancel()
+
+				return
 			}
 		}
 	}