extensions: libxt_devgroup: handle the invert flag properly in translation

extensions: libxt_devgroup: handle the invert flag properly in translation

We forgot to put "!=" when devgroup can be mapped to name, so translation
is wrong:
  # iptables-translate -A OUTPUT -m devgroup ! --dst-group 0
  nft add rule ip filter OUTPUT oifgroup default counter

Apply this patch:
  # iptables-translate -A OUTPUT -m devgroup ! --dst-group 0
  nft add rule ip filter OUTPUT oifgroup != default counter

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>