miniupnpc: update Changelog.txt
regarding
$Id: Changelog.txt,v 1.247 2020/12/20 18:07:34 nanard Exp $
miniUPnP client Changelog.
+2020/12/30:
+ Fix usage of IP_MULTICAST_IF with struct ip_mreqn
+
VERSION 2.2.1 : released 2020/12/20
2020/11/30:
