gtphub: add to debian build

By the example of osmo-sgsn, package osmo-gtphub for debian.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg b/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg
new file mode 100644
index 0000000..c9bb4cf
--- /dev/null
+++ b/openbsc/doc/examples/osmo-gtphub/osmo-gtphub.cfg
@@ -0,0 +1,23 @@
+!
+! Osmocom gtphub configuration
+!
+
+line vty
+ no login
+
+gtphub
+ ! Local addresses to listen on and send from, each on standard ports
+ ! 2123 and 2152. Setting these addresses is mandatory.
+ bind-to-sgsns 127.0.0.1
+ bind-to-ggsns 127.0.0.2
+
+ ! Local nonstandard ports or separate IPs:
+ !bind-to-sgsns ctrl 127.0.0.1 2342 user 127.0.0.1 4223
+
+ ! Proxy: unconditionally direct all traffic to...
+ !ggsn-proxy 127.0.0.3
+ !sgsn-proxy 127.0.0.4
+
+ ! Proxy with nonstandard ports or separate IPs:
+ !ggsn-proxy ctrl 127.0.0.3 2123 user 127.0.0.5 2152
+