Buildrequire systemd for rpm macros

See merge request !1

Steve Traylen authored Jan 17, 2020 and Ben Morrice committed Jan 17, 2020

With cern-aklog-systemd-user-1.0-1.el8.cern there are unxpanded macros

```
rpm -ql cern-aklog-systemd-user
/%{_userunitdir}/aklog.service
```

and

```
postinstall scriptlet (using /bin/sh):
%systemd_user_post aklog.service
preuninstall scriptlet (using /bin/sh):
%systemd_user_preun aklog.service
postuninstall scriptlet (using /bin/sh):
%systemd_user_postun_with_restart aklog.service
```

Buildrequire systemd to ensure they evaluated at rpm build time.