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

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

(From meta-openembedded rev: d096e77a362aba8bd70fc02413c16135bace7f7f)

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>