[kcms/cursortheme] Don't skip everything because of lack of XFixes

Summary:
XFixes is only used on X.
But we definitely still need to update the available entries in the size
combo box.
We still want to emit changes, and we still want to update the env

Test Plan: Ran on wayland

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27055