netfilter: nfnl_hook: fix unused variable warning

The only user of this variable is in an #ifdef:

net/netfilter/nfnetlink_hook.c: In function 'nfnl_hook_entries_head':
net/netfilter/nfnetlink_hook.c:177:28: error: unused variable 'netdev' [-Werror=unused-variable]

Fixes: e2cf17d3 ("netfilter: add new hook nfnl subsystem")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_hook.c

@@ -174,7 +174,9 @@ static const struct nf_hook_entries *
 nfnl_hook_entries_head(u8 pf, unsigned int hook, struct net *net, const char *dev)
 {
 	const struct nf_hook_entries *hook_head = NULL;
+#ifdef CONFIG_NETFILTER_INGRESS
 	struct net_device *netdev;
+#endif
 
 	switch (pf) {
 	case NFPROTO_IPV4: