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.examples b/debian/osmo-gtphub.examples
new file mode 100644
index 0000000..48c2dc0
--- /dev/null
+++ b/debian/osmo-gtphub.examples
@@ -0,0 +1 @@
+openbsc/doc/examples/osmo-gtphub