[packaging] BuildRequire systemd via pkgconfig. JB#55010
[packaging] BuildRequire systemd via pkgconfig. JB#55010

This allows to pick systemd-mini inside the builder instead of full
systemd.

Signed-off-by: Björn Bidar <98956b378934db91ace7f37757fdd09a74df421e@jolla.com>

sailfishos/sailfish-polkit-agent,sailfishos/sailfish-polkit-agent