smack: smk_ipv6_port_list should be static

Fixes the following sparse warning:

 security/smack/smack_lsm.c:55:1: warning: symbol 'smk_ipv6_port_list'
 was not declared. Should it be static?
Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -52,7 +52,7 @@
 #define SMK_SENDING	2
 
 #ifdef SMACK_IPV6_PORT_LABELING
-LIST_HEAD(smk_ipv6_port_list);
+static LIST_HEAD(smk_ipv6_port_list);
 #endif
 static struct kmem_cache *smack_inode_cache;
 int smack_enabled;