sddm: Always put sddm.service in the correct location

Summary:
To avoid errors if using the usrmerge DISTRO_FEATURE let's always put
the sddm.service file in the systemd_unitdir path.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

Reviewers: vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D22744