netfilter: br_netfilter: update stub br_nf_pre_routing_ipv6 parameter to `void *priv`.

The real br_nf_pre_routing_ipv6 function, defined when CONFIG_IPV6 is
enabled, expects `void *priv`, not `const struct nf_hook_ops *ops`.
Update the stub br_nf_pre_routing_ipv6, defined when CONFIG_IPV6 is
disabled, to match.

Fixes: 06198b34 ("netfilter: Pass priv instead of nf_hook_ops to netfilter hooks")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>