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: NZhao Hongjiang <zhaohongjiang@huawei.com>
Acked-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cpuset.c

@@ -70,7 +70,6 @@ int number_of_cpusets __read_mostly;
 
 /* Forward declare cgroup structures */
 struct cgroup_subsys cpuset_subsys;
-struct cpuset;
 
 /* See "Frequency meter" comments, below. */