kernel: nf-nathelper-extra depends on ipt-raw

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As automatic helper assignment is disabled in recent Linux kernels,
explicit rules must be added to the raw table for each helper.

While commit f50a524 in the firewall3 project added a set of default
rules and other additional related functionality, both this and the
alternative manual methods of defining these rules require kmod-ipt-raw.

Signed-off-by: Steven Honson <steven@honson.id.au>