if no xcursor is found, don't build the kcm at all

Summary:
the code path of kcm without xcursors was broken since over a year,
meaning is not tested at all and can't be guaranteed to be supported

Test Plan: ino build error when xcursor is not found

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20375