osmo-gtphub.cfg: fix conflict with osmo-ggsn.cfg

Change bind-to-ggsns from 127.0.0.2 to 127.0.0.20, so osmo-gtphub's
default config does not confligt with the osmo-ggsn default config. This
change is for the effort of making the configs of all Osmocom programs
not conflict with each other.

A similar change was made in Id892e1f4ab2daabbe9824b819b5fed985373b97a
with bind-to-sgsns.

Related: OS#5817
Change-Id: I57ee457b62139d831707b6ebd6baaea8d33c2d9c

doc/examples/osmo-gtphub/osmo-gtphub.cfg

@@ -9,7 +9,7 @@ gtphub
  ! Local addresses to listen on and send from, each on standard ports
  ! 2123 and 2152. Setting these addresses is mandatory.
  bind-to-sgsns 127.0.0.10
- bind-to-ggsns 127.0.0.2
+ bind-to-ggsns 127.0.0.20
 
  ! Local nonstandard ports or separate IPs:
  !bind-to-sgsns ctrl 127.0.0.1 2342 user 127.0.0.1 4223