tailcfg: add SetDNSResponse

Signed-off-by: Maisem Ali <maisem@tailscale.com>

control/controlclient/direct.go

@@ -1298,7 +1298,7 @@ func (c *Direct) SetDNS(ctx context.Context, req *tailcfg.SetDNSRequest) (err er
 		msg, _ := ioutil.ReadAll(res.Body)
 		return fmt.Errorf("set-dns response: %v, %.200s", res.Status, strings.TrimSpace(string(msg)))
 	}
-	var setDNSRes struct{} // no fields yet
+	var setDNSRes tailcfg.SetDNSResponse
 	if err := decode(res, &setDNSRes, serverKey, machinePrivKey); err != nil {
 		c.logf("error decoding SetDNSResponse with server key %s and machine key %s: %v", serverKey, machinePrivKey.Public(), err)
 		return fmt.Errorf("set-dns-response: %v", err)

tailcfg/tailcfg.go

@@ -1534,6 +1534,9 @@ type SetDNSRequest struct {
 	Value string
 }
 
+// SetDNSResponse is the response to a SetDNSRequest.
+type SetDNSResponse struct{}
+
 // SSHPolicy is the policy for how to handle incoming SSH connections
 // over Tailscale.
 type SSHPolicy struct {