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: I7696cd92953787591e7b4777ee79c2671326b972
diff --git a/contrib/systemd/osmo-mgw.service b/contrib/systemd/osmo-mgw.service
index f75277e..5798491 100644
--- a/contrib/systemd/osmo-mgw.service
+++ b/contrib/systemd/osmo-mgw.service
@@ -3,6 +3,8 @@
 
 [Service]
 Type=simple
+StateDirectory=osmocom
+WorkingDirectory=%S/osmocom
 Restart=always
 ExecStart=/usr/bin/osmo-mgw -s -c /etc/osmocom/osmo-mgw.cfg
 RestartSec=2