seatd: fix packaging error with systemd DISTRO_FEATURES

seatd: fix packaging error with systemd DISTRO_FEATURES

The systemd unit file for this recipe is only installed in case
the target system's init system is systemd. It is achieved by
inspecting the VIRTUAL-RUNTIME_init_manager variable.

However the systemd class was inherited unconditionally. This caused
a failure in do_package task, in case systemd is present in the
DISTRO_FEATURES but the system's init manager is sysvinit: in this case
the systemd unit file is not installed, however systemd.bbclass is
still trying to register is as a startup service. At this point
it failed:

ERROR: seatd-0.9.1-r0 do_package: Didn't find service unit 'seatd.service', specified in SYSTEMD_SERVICE:seatd.

To avoid this, install the systemd unit file unconditionally, regardless
of the used init system.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>