osmoappdesc.py: fix wrong index introduced by gtphub.
vty_app should reference osmo-nitb, but by adding gtphub, apps[-1] has
changed. Use index 5, which won't change when adding further items.
diff --git a/openbsc/osmoappdesc.py b/openbsc/osmoappdesc.py
index 67b0d35..c21883b 100644
--- a/openbsc/osmoappdesc.py
+++ b/openbsc/osmoappdesc.py
@@ -52,4 +52,4 @@
vty_command = ["./src/osmo-nitb/osmo-nitb", "-c",
"doc/examples/osmo-nitb/nanobts/openbsc.cfg"]
-vty_app = apps[-1]
+vty_app = apps[5] # reference apps[] entry for osmo-nitb