netfilter: ctnetlink: add const qualifier to nfnl_hook.get_ct

get_ct as is and will not update its skb argument, and users of
nfnl_ct_hook is currently only nfqueue, we can add const qualifier.

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 71d6ada..9f52729 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -2162,7 +2162,7 @@
 	       ;
 }
 
-static struct nf_conn *ctnetlink_glue_get_ct(struct sk_buff *skb,
+static struct nf_conn *ctnetlink_glue_get_ct(const struct sk_buff *skb,
 					     enum ip_conntrack_info *ctinfo)
 {
 	struct nf_conn *ct;