powerpc/mm/hugetlb: Make pseries_alloc_bootmem_huge_page() static

pseries_alloc_bootmem_huge_page() is only used locally in
alloc_bootmem_huge_page() and does not need to be external.

It fixes this W=1 compile error :

../arch/powerpc/mm/hugetlbpage.c:220:12: error: no previous prototype for ‘pseries_alloc_bootmem_huge_page’ [-Werror=missing-prototypes]
  220 | int __init pseries_alloc_bootmem_huge_page(struct hstate *hstate)
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-16-clg@kaod.org