Fix anchors error in AbstractKCM types
This is just the way Kirigami.Page works, and there's only so much we can do in downstream components.
Fixes issues like the following:
file:///usr/local/kde/share/kpackage/kcms/kcm_sddm/contents/ui/main.qml:22:13: QML InlineMessage: Cannot anchor to an item that isn't a parent or sibling.