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-bsc.service b/openbsc/contrib/systemd/osmo-bsc.service
index deaf123..eb6cacb 100644
--- a/openbsc/contrib/systemd/osmo-bsc.service
+++ b/openbsc/contrib/systemd/osmo-bsc.service
@@ -7,3 +7,6 @@
 Restart=always
 ExecStart=/usr/bin/osmo-bsc -c /etc/osmocom/osmo-bsc.cfg -s
 RestartSec=2
+
+[Install]
+WantedBy=multi-user.target