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>