netfilter: xt_LOG: add missing string format in nf_log_packet()

   net/netfilter/xt_LOG.c: In function 'log_tg':
>> net/netfilter/xt_LOG.c:43: error: format not a string literal and no format arguments

Fixes: fab4085f ("netfilter: log: nf_log_packet() as real unified interface")
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_LOG.c

@@ -40,7 +40,7 @@ log_tg(struct sk_buff *skb, const struct xt_action_param *par)
 	li.u.log.logflags = loginfo->logflags;
 
 	nf_log_packet(net, par->family, par->hooknum, skb, par->in, par->out,
-		      &li, loginfo->prefix);
+		      &li, "%s", loginfo->prefix);
 	return XT_CONTINUE;
 }