net_sched: sch_plug: plug_qdisc_ops is static

net/sched/sch_plug.c:211:18: warning: symbol 'plug_qdisc_ops' was not
declared. Should it be static?

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>