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