mm: page_alloc: make compound_page_dtors static

It's only used inside page_alloc.c now. So make it static and remove the
declaration in mm.h.

Link: https://lkml.kernel.org/r/20230617034622.1235913-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>