cmd/tailscale: fix regression from earlier commit

In 00a42702cb I bumped go.mod and adjusted the API, but only tested
that it compiled & tests (hah) passed.

I forgot this line. I still haven't tested this, but can't be worse! :)

Fixes 00a42702cb (commitcomment-112642284)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/backend.go

@@ -135,6 +135,7 @@ func newBackend(dataDir string, jvm *jni.JVM, appCtx jni.Object, store *stateSto
 		Router:       cb,
 		DNS:          cb,
 		Dialer:       dialer,
+		SetSubsystem: sys.Set,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("runBackend: NewUserspaceEngine: %v", err)