sh: pgtable: Fix missing prototypes

sh: pgtable: Fix missing prototypes

arch/sh/mm/pgtable.c:12:6: warning: no previous prototype for 'pgd_ctor' [-Wmissing-prototypes]
arch/sh/mm/pgtable.c:34:8: warning: no previous prototype for 'pgd_alloc' [-Wmissing-prototypes]
arch/sh/mm/pgtable.c:39:6: warning: no previous prototype for 'pgd_free' [-Wmissing-prototypes]
arch/sh/mm/pgtable.c:45:6: warning: no previous prototype for 'pud_populate' [-Wmissing-prototypes]
arch/sh/mm/pgtable.c:50:8: warning: no previous prototype for 'pmd_alloc_one' [-Wmissing-prototypes]
arch/sh/mm/pgtable.c:55:6: warning: no previous prototype for 'pmd_free' [-Wmissing-prototypes]

Make pgd_ctor() static, as it is only used in this file.
Include <asm/pgalloc.h> to fix the other warnings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/aa1328ea3327d6685aab76d5787cb77c996052c0.1709326528.git.geert+renesas@glider.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>