systemd: Add Install section to service files

That way the services can be enabled with
> systemctl enable <service>
diff --git a/openbsc/contrib/systemd/osmo-nitb.service b/openbsc/contrib/systemd/osmo-nitb.service
index a9d5d8c..377497e 100644
--- a/openbsc/contrib/systemd/osmo-nitb.service
+++ b/openbsc/contrib/systemd/osmo-nitb.service
@@ -6,3 +6,6 @@
 Restart=always
 ExecStart=/usr/bin/osmo-nitb -s -C -c /etc/osmocom/osmo-nitb.cfg -l /var/lib/osmocom/hlr.sqlite3
 RestartSec=2
+
+[Install]
+WantedBy=multi-user.target