netfilter: nf_nat_snmp_basic: add missing helper alias name

In order to upload helper module automatically, helper alias name
is needed. so that MODULE_ALIAS_NFCT_HELPER() should be added.
And unlike other nat helper modules, the nf_nat_snmp_basic can be
used independently.
helper name is "snmp_trap" so that alias name will be
"nfct-helper-snmp_trap" by MODULE_ALIAS_NFCT_HELPER(snmp_trap)

test command:
   %iptables -t raw -I PREROUTING -p udp -j CT --helper snmp_trap
   %lsmod | grep nf_nat_snmp_basic

We can see nf_nat_snmp_basic module is uploaded automatically.
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_nat_snmp_basic_main.c

@@ -60,6 +60,7 @@ MODULE_LICENSE("GPL");
 MODULE_AUTHOR("James Morris <jmorris@intercode.com.au>");
 MODULE_DESCRIPTION("Basic SNMP Application Layer Gateway");
 MODULE_ALIAS("ip_nat_snmp_basic");
+MODULE_ALIAS_NFCT_HELPER("snmp_trap");
 
 #define SNMP_PORT 161
 #define SNMP_TRAP_PORT 162