net: sched: make function qdisc_free_cb() static

Fixes the following sparse warning:

net/sched/sch_generic.c:944:6: warning:
 symbol 'qdisc_free_cb' was not declared. Should it be static?

Fixes: 3a7d0d07 ("net: sched: extend Qdisc with rcu")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -941,7 +941,7 @@ void qdisc_free(struct Qdisc *qdisc)
 	kfree((char *) qdisc - qdisc->padded);
 }
 
-void qdisc_free_cb(struct rcu_head *head)
+static void qdisc_free_cb(struct rcu_head *head)
 {
 	struct Qdisc *q = container_of(head, struct Qdisc, rcu);