cpuset: fix non static symbol warning

Fixes the following sparse warning:

kernel/cpuset.c:2088:6: warning:
 symbol 'cpuset_fork' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>