LockAssistant is a Cydia Tweak which will add a Siri button on your Lockscreen. It does this by always showing your Camera button you would normally see when double-tapping the home button. Sometimes it can get in the way or you'll accidentally press it. It would be great if there was an option to not always show the button, so hopefully we'll see an update. You can find LockAssistant in the Cydia Store, for FREE, under the Big Boss Repo.