gtphub: add to debian build

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

Sponsored-by: On-Waves ehi
diff --git a/debian/osmo-gtphub.default b/debian/osmo-gtphub.default
new file mode 100644
index 0000000..6af82da
--- /dev/null
+++ b/debian/osmo-gtphub.default
@@ -0,0 +1,2 @@
+CONFIG_FILE="/etc/osmocom/osmo-gtphub.cfg"
+