cpuset: get rid of the useless forward declaration of cpuset

get rid of the useless forward declaration of the struct cpuset cause the 
below define it.

Signed-off-by: Zhao Hongjiang <zhaohongjiang@huawei.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>