Require out-of-source build

Summary: KService doesn't build if you try to do an in-source build (fakeplugin.desktop gets renamed in the wrong place and CMake dies), so let's just require an out-of-source build, which people should be doing anyway.

Test Plan:

• Builds with an out-of-source build
• Stops immediately if you try to do an in-source build

Reviewers: dfaure, cfeck, Frameworks

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14003