net_sched: nla_memdup_cookie() can be static

Fixes the following sparse warning:

net/sched/act_api.c:532:5: warning:
 symbol 'nla_memdup_cookie' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>