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-mgcp.service b/openbsc/contrib/systemd/osmo-bsc-mgcp.service
index 7fa2ec0..c040e60 100644
--- a/openbsc/contrib/systemd/osmo-bsc-mgcp.service
+++ b/openbsc/contrib/systemd/osmo-bsc-mgcp.service
@@ -6,3 +6,6 @@
 Restart=always
 ExecStart=/usr/bin/osmo-bsc_mgcp -s -c /etc/osmocom/osmo-bsc-mgcp.cfg
 RestartSec=2
+
+[Install]
+WantedBy=multi-user.target