[NETFILTER]: Fix NAT init order

As noticed by Phil Oester, the GRE NAT protocol helper is initialized
before the NAT core, which makes registration fail.

Change the linking order to make NAT be initialized first.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>