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>

net/netfilter/nf_conntrack_snmp.c

@@ -16,6 +16,7 @@
 #include <net/netfilter/nf_conntrack.h>
 #include <net/netfilter/nf_conntrack_helper.h>
 #include <net/netfilter/nf_conntrack_expect.h>
+#include <linux/netfilter/nf_conntrack_snmp.h>
 
 #define SNMP_PORT	161