mm/memory_hotplug: add static qualifier for online_policy_to_str()

online_policy_to_str is only used in memory_hotplug.c and should be
defined as static.

Link: https://lkml.kernel.org/r/20210913024534.26161-1-tangyizhou@huawei.com


Signed-off-by: Tang Yizhou <tangyizhou@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>