Use the ntp support for 'systemd-timesyncd.service' too

With systemd 217 in Ubuntu, and systemd-shim 9:

laney@raleigh> gdbus call -y -d org.freedesktop.timedate1 -o /org/freedesktop/timedate1 -m org.freedesktop.timedate1.SetNTP true false
Error: GDBus.Error:org.freedesktop.DBus.Error.FileNotFound: Unknown unit: systemd-timesyncd.service
(According to introspection data, you need to pass 'bb')
laney@raleigh> timedatectl
[…]
     NTP enabled: n/a
[…]

but with the new one

laney@raleigh> timedatectl
[…]
     NTP enabled: yes