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

arch/powerpc/include/asm/book3s/64/mmu-hash.h

@@ -467,6 +467,8 @@ extern int hash_page_mm(struct mm_struct *mm, unsigned long ea,
 			unsigned long flags);
 extern int hash_page(unsigned long ea, unsigned long access, unsigned long trap,
 		     unsigned long dsisr);
+void low_hash_fault(struct pt_regs *regs, unsigned long address, int rc);
+int __hash_page(unsigned long trap, unsigned long ea, unsigned long dsisr, unsigned long msr);
 int __hash_page_huge(unsigned long ea, unsigned long access, unsigned long vsid,
 		     pte_t *ptep, unsigned long trap, unsigned long flags,
 		     int ssize, unsigned int shift, unsigned int mmu_psize);