Use XCursorTheme on wayland as well
Summary: wayland-cursor uses xcursor to load cursors, so this is fine.
Test Plan: I can select cursors on wayland again.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D12361