TT#105868 Depend on cron in ngcp-services-{ce,pro}

We depend on a working cron service in ngcp-installer:

| 2020-12-17 22:07:16 spce: Synchronizing current with expected services state
| Failed to start cron.service: Unit cron.service not found.
| +22:07:17 (cfg.inc:450): cfg_apply_configs_default(): log_die 'Can not apply configuration'
| +22:07:17 (log.inc:27): log_die(): echo 'ABORTED: Can not apply configuration'
| ABORTED: Can not apply configuration

We don't have an explicit dependency on cron though:

| $ aptitude why cron
| i   ngcp-ngcp-ce     Depends ngcp-services-ce
| i A ngcp-services-ce Depends logrotate
| i A logrotate        Depends cron | anacron | cron-daemon | systemd-sysv

We have systemd-sysv present, so cron isn't necessarily pulled in any
longer automatically (at least with apt's resolver as present in current
Debian/bullseye). So explicitly depend on cron package.

Change-Id: I3a46f853dfcfca5a0737553eaa35ffbf8342a7cf

debian/control

@@ -285,6 +285,7 @@ Depends:
  acpid,
  bsd-mailx | mailx,
  bzip2,
+ cron,
  dbus,
  ethtool,
  exim4,
@@ -395,6 +396,7 @@ Depends:
  acpid,
  bsd-mailx | mailx,
  bzip2,
+ cron,
  dbus,
  ethtool,
  grub-efi | grub-pc,