pkt_sched: fix sparse warning

Impact: make global function static

Fix the following sparse warning:

  net/sched/sch_api.c:192:14: warning: symbol 'qdisc_match_from_root' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>