PKGBUILD: Add systemd to depends

systemd is required at runtime and we also need it at build time.
New chroots created by Arch's devtools no longer include all base
packages and systemd won't be installed unless we depend on it.

Without systemd the makefile won't find the unit dir and will install
the service files into /.

Signed-off-by: Florian Pritz <bluewind@xinu.at>