systemd: use Wants for the dependency btw. osmo-bsc-mgcp and osmo-bsc

* osmo-bsc and osmo-bsc-mgcp are needed to run
* with Wants, both are started but can be indepently
  controlled via systemctl

Fixes: SYS#738
diff --git a/openbsc/contrib/systemd/osmo-bsc.service b/openbsc/contrib/systemd/osmo-bsc.service
index eb6cacb..4047fef 100644
--- a/openbsc/contrib/systemd/osmo-bsc.service
+++ b/openbsc/contrib/systemd/osmo-bsc.service
@@ -1,6 +1,6 @@
 [Unit]
 Description=OpenBSC BSC
-Requires=osmo-bsc-mgcp.service
+Wants=osmo-bsc-mgcp.service
 
 [Service]
 Type=simple