Create KSelectionOwner in StartupFeedbackEffect only on X11

Summary:
On Wayland we do not need the selection. By only creating the
KSelectionOwner on X11 an X11-runtime dependency is removed from the
StartupFeedbackEffect.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3121