netfilter: xtables: remove duplicate member

Accidentally missed removing the old out-of-union "inverse" member,
which caused the struct size to change which then gives size mismatch
warnings when using an old iptables.

It is interesting to see that gcc did not warn about this before.
(Filed http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47376 )

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>