tailcfg: make MapResponse.ControlTime a pointer

Otherwise omitempty doesn't work.

This is wire-compatible with a non-pointer type, so switching
is safe, now and in the future.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tailcfg/tailcfg.go

@@ -1297,7 +1297,7 @@ type MapResponse struct {
 	SSHPolicy *SSHPolicy `json:",omitempty"`
 
 	// ControlTime, if non-zero, is the current timestamp according to the control server.
-	ControlTime time.Time `json:",omitempty"`
+	ControlTime *time.Time `json:",omitempty"`
 
 	// Debug is normally nil, except for when the control server
 	// is setting debug settings on a node.