hugetlbfs: remove unneeded header file

The header file signal.h is unneeded now. Remove it.

Link: https://lkml.kernel.org/r/20220726142918.51693-4-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>