etc/nsca-ng.service: Depend on socket activation

The nsca-ng(8) server currently has no proper support for running under
systemd(1) without socket activation.  Therefore, the systemd.service(5)
should explicitly depend on the systemd.socket(5):

| If you want to make sure your service never tries to start without
| socket activation, it should have Requires=foo.socket; none of the
| default relations are strong enough to strictly prohibit starting
| without a socket.  I think at least the case where creating the socket
| fails and admin manually says "systemctl start foo.service" would
| always start the service without a socket otherwise.

[ http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=1689;bug=727708 ]