desktop/notification: stay compatble with go < 1.13

time.Duration.Milliseconds was added in go 1.13, and has a trivial
implementation. Replicate that implementation instead of depending on
it.

Ref: https://go-review.googlesource.com/c/go/+/167387/2/src/time/time.go
Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

desktop/notification/fdo.go

@@ -80,7 +80,7 @@ func (srv *Server) SendNotification(msg *Message) (ID, error) {
 	call := srv.obj.Call(dBusInterfaceName+".Notify", 0,
 		msg.AppName, msg.ReplacesID, msg.Icon, msg.Summary, msg.Body,
 		flattenActions(msg.Actions), mapHints(msg.Hints),
-		int32(msg.ExpireTimeout.Milliseconds()))
+		int32(msg.ExpireTimeout.Nanoseconds()/1e6))
 	var id ID
 	if err := call.Store(&id); err != nil {
 		return 0, err