MAINTAINERS: netfilter: add irc channel

The community #netfilter IRC channel is now live on the libera.chat network
(https://libera.chat/).

CC: Arturo Borrero Gonzalez <arturo@netfilter.org>
Link: https://marc.info/?l=netfilter&m=162210948632717


Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>