netfilter: fix crash during boot if NAT has been compiled built-in

(c7232c99 netfilter: add protocol independent NAT core) introduced a
problem that leads to crashing during boot due to NULL pointer
dereference. It seems that xt_nat calls xt_register_target() before
xt_init():

net/netfilter/x_tables.c:static struct xt_af *xt; is NULL and we crash on
xt_register_target(struct xt_target *target)
{
        u_int8_t af = target->family;
        int ret;

        ret = mutex_lock_interruptible(&xt[af].mutex);
...

Fix this by changing the linking order, to make sure that x_tables
comes before xt_nat.

Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>