kcms/ksmserver: Fix settings object lifetime management
Registering a QML singleton with an object with a small lifespan than the engine is wrong.
This will assert (or at best be broken) when the KCM is opened, closed and reopened.
This is in line with other KCMs.
(cherry picked from commit 5d04608f)
Co-authored-by: David Edmundson kde@davidedmundson.co.uk