The make logic assumes that the SYSTEMD_UNIT_FILES var can be built from
SYSTEMD_UNIT_FILES_IN by simply dropping the directory prefix and the
.in suffix.

This won't work in future when a single .in unit file can be used to
generate multiple different units.
Reviewed-by: NAndrea Bolognani <abologna@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>