Enable systemd support by compiling against libsystemd-dev

By adding libsystemd-dev and pkg-config to Build-Depends,
the build system automatically enables systemd support.

Only when systemd support is explicitly enabled, redis versions
>=6.0.9-1 provide support for sd_notify() / NOTIFY_SOCKET / READY=1,
otherwise the service is stuck in `activating` when used with
`Type=notify`.

Closes: #977852