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: 5caaed151a68 ("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>

net/netfilter/nf_conntrack_netlink.c

@@ -533,6 +533,7 @@ nla_put_failure:
 	return -1;
 }
 
+#if defined(CONFIG_NETFILTER_NETLINK_GLUE_CT) || defined(CONFIG_NF_CONNTRACK_EVENTS)
 static size_t ctnetlink_proto_size(const struct nf_conn *ct)
 {
 	const struct nf_conntrack_l3proto *l3proto;
@@ -552,6 +553,7 @@ static size_t ctnetlink_proto_size(const struct nf_conn *ct)
 
 	return len + len4;
 }
+#endif
 
 static inline size_t ctnetlink_acct_size(const struct nf_conn *ct)
 {