Fix crash in KCMKeyboardWidget::populateWithCurrentXkbOptions on Wayland

Summary:
Prevents a call into X11 which will crash. This needs to be fixed
properly by defining a way to get the options from the Wayland
compositor.

Test Plan: Can open the KCM without crash

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6581