powerpc/mm: Declare some prototypes

It fixes this W=1 compile error :

../arch/powerpc/mm/book3s64/hash_utils.c:1515:5: error: no previous prototype for ‘__hash_page’ [-Werror=missing-prototypes]
 1515 | int __hash_page(unsigned long trap, unsigned long ea, unsigned long dsisr,
      |     ^~~~~~~~~~~
../arch/powerpc/mm/book3s64/hash_utils.c:1850:6: error: no previous prototype for ‘low_hash_fault’ [-Werror=missing-prototypes]
 1850 | void low_hash_fault(struct pt_regs *regs, unsigned long address, int rc)
      |      ^~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-13-clg@kaod.org