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