check for systemd before running the timesyncd exit hook.

[perkelix]

As per Debian bug, it would be desirable to start the timesyncd hook with something that checks for the presence of timesyncd and, if NOT found, immediately exits the hook.

Debian's dhclient exit hook might provide a good starting point:

if [ ! -d /run/systemd/system ]; then
	return
fi
if [ ! -x /lib/systemd/systemd-timesyncd ]; then
	return
fi