Add install dir for systemd units

Summary: Like we have for other FD.O projects like DBus service files

Test Plan:
Unit test passes
Used in another project, and it installed to the correct place

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D25107