mm/vmstat: remove unused page_ext.h from vmstat

No page_ext function or structure is used in vmstat.  Just remove page_ext
header from vmstat.

Link: https://lkml.kernel.org/r/20230717113227.1897173-3-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>