Set working directory in systemd service file

By default systemd will execute service with root directory (or home directory for user instance) which might result in
attempts to create files in unexpected place. Let's set it to 'osmocom' subdir of state directory (/var/lib for system instance) instead.

Related: OS#4821
Change-Id: I5117ab8d7b8542a161f69a9784a9667bda599473
diff --git a/contrib/systemd/osmo-gbproxy.service b/contrib/systemd/osmo-gbproxy.service
index a0b7829..6655d91 100644
--- a/contrib/systemd/osmo-gbproxy.service
+++ b/contrib/systemd/osmo-gbproxy.service
@@ -4,6 +4,8 @@
 [Service]
 Type=simple
 ExecStart=/usr/bin/osmo-gbproxy -c /etc/osmocom/osmo-gbproxy.cfg
+StateDirectory=osmocom
+WorkingDirectory=%S/osmocom
 Restart=always
 RestartSec=2
 RestartPreventExitStatus=1