pkt_sched: Fix build with NET_SCHED disabled.

The stab bits can't be referenced uniless the full
packet scheduler layer is enabled.

Reported by Stephen Rothwell.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -469,7 +469,9 @@ static void __qdisc_destroy(struct rcu_head *head)
 	struct Qdisc *qdisc = container_of(head, struct Qdisc, q_rcu);
 	const struct Qdisc_ops  *ops = qdisc->ops;
 
+#ifdef CONFIG_NET_SCHED
 	qdisc_put_stab(qdisc->stab);
+#endif
 	gen_kill_estimator(&qdisc->bstats, &qdisc->rate_est);
 	if (ops->reset)
 		ops->reset(qdisc);