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