package/squid: add optional systemd dependency

systemd is an optional dependency (enabled by default) since version
4.11 and
https://github.com/squid-cache/squid/commit/6fa8c66435d55a2e713db0222cdca3a9dccf5bbe



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>