iptables/src/libxt_ifgroup.c: Drop calls to xtables_check_inverse()

xtables_check_inverse() is non-existent in recent iptables, drop calls
to it.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #2 from algernon/h/iptables/xt_ifgroup/no-check-inverse

iptables/src/libxt_ifgroup.c: Drop calls to xtables_check_inverse()