netfilter: xt_CT: remove a compile warning

If CONFIG_NF_CONNTRACK_TIMEOUT=n we have following warning :

  CC [M]  net/netfilter/xt_CT.o
net/netfilter/xt_CT.c: In function ‘xt_ct_tg_check_v1’:
net/netfilter/xt_CT.c:284: warning: label ‘err4’ defined but not used

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_CT.c

@@ -281,8 +281,10 @@ out:
 	info->ct = ct;
 	return 0;
 
+#ifdef CONFIG_NF_CONNTRACK_TIMEOUT
 err4:
 	rcu_read_unlock();
+#endif
 err3:
 	nf_conntrack_free(ct);
 err2: