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>