net: sched: move rtm_tca_policy declaration to include file

rtm_tca_policy is used from net/sched/sch_api.c and net/sched/cls_api.c,
thus should be declared in an include file.

This fixes the following sparse warning:
net/sched/sch_api.c:1434:25: warning: symbol 'rtm_tca_policy' was not declared. Should it be static?

Fixes: e331473f ("net/sched: cls_api: add missing validation of netlink attributes")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>