netfilter: nf_ct_snmp: add include file

Prototype for nf_nat_snmp_hook is in nf_conntrack_snmp.h therefore
it should be included to get type checking. Found by sparse.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>