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.install b/debian/osmo-gtphub.install
new file mode 100644
index 0000000..908c1a5
--- /dev/null
+++ b/debian/osmo-gtphub.install
@@ -0,0 +1 @@
+/usr/bin/osmo-gtphub