ila: add NETFILTER dependency

The recently added generic ILA translation facility fails to
build when CONFIG_NETFILTER is disabled:

net/ipv6/ila/ila_xlat.c:229:20: warning: 'struct nf_hook_state' declared inside parameter list
net/ipv6/ila/ila_xlat.c:235:27: error: array type has incomplete element type 'struct nf_hook_ops'
 static struct nf_hook_ops ila_nf_hook_ops[] __read_mostly = {

This adds an explicit Kconfig dependency to avoid that case.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 7f00feaf

 ("ila: Add generic ILA translation facility")
Signed-off-by: David S. Miller <davem@davemloft.net>