seatd: build systemd backend if DISTRO_FEATURE systemd is set

Add libsystemd dependency if we have systemd in DISTRO_FEATURES.
This is needed to build the systemd backend. Projects that use seatd
to hook into logind (e.g. wlroots) fail to properly login without it.

Signed-off-by: Markus Volk <f_l_k@t-online.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>