Changes

rebar.lock: update gtplib commit

Let the commit point at the tree that was just rebased, so building
with a newer erlang version on debian unstable is fixed.

Fix for:
  [  619s] ===> Compiling _checkouts/gtplib/src/gtp_packet.erl failed
  [  619s] gtp_packet.erl:252:1: syntax error before: 'maybe'

Fixes: OS#6661
Change-Id: I44bd92b9d9ca90fe396d85f67a064cba08b3f6c3