Add missing return in shutdown
See https://github.com/juanfont/headscale/issues/1461

Index: hscontrol/app.go
--- hscontrol/app.go.orig
+++ hscontrol/app.go
@@ -821,6 +821,8 @@ func (h *Headscale) Serve() error {
 
 				// And we're done:
 				cancel()
+
+				return
 			}
 		}
 	}
