ntp: Fix the installed but not shipped issue of a systemd units file

We will get the following ERROR/WARN if we enable systemd and
the installed-vs-shipped check in QA:

ERROR: QA Issue: ntp: Files/directories were installed but not shipped
  /lib/systemd/ntp-units.d
  /lib/systemd/ntp-units.d/60-ntpd.list

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>