netfilter: add ifdef around ctnetlink_proto_size

This function is no longer marked 'inline', so we now get a warning
when it is unused:

net/netfilter/nf_conntrack_netlink.c:536:15: error: 'ctnetlink_proto_size' defined but not used [-Werror=unused-function]

We could mark it inline again, mark it __maybe_unused, or add an #ifdef
around the definition. I'm picking the third approach here since that
seems to be what the rest of the file has.

Fixes: 5caaed15 ("netfilter: conntrack: don't cache nlattr_tuple_size result in nla_size")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>