Changes

icmpv6: Re-license code under GPL-v2-or-later

The original statement in my code from osmo-ggsn commit
d46bcd236e93432c894a939f4e5810dc5e9b4974 in 2017 was worded GPLv2-only,
which was most likely an oversight.

All subsequent modifications to this code were done by either me or Pau
Espin, who was doing the work under contract with sysmocom, of which I'm
the managing director.  Therefore, I have legal authority to re-license
this file to v2-or-later.

Change-Id: I5a4fa18ef64c2f228e21fa2c067e55eaa4080508