[NETFILTER]: NAT: silence unused variable warnings with CONFIG_XFRM=n

net/ipv4/netfilter/ip_nat_standalone.c: In function 'ip_nat_out':
net/ipv4/netfilter/ip_nat_standalone.c:223: warning: unused variable 'ctinfo'
net/ipv4/netfilter/ip_nat_standalone.c:222: warning: unused variable 'ct'

Surprisingly no complaints so far ..

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