powerpc/mm: fix missing prototypes in slice.c

This patch fixes the following warnings (obtained with make W=1).

arch/powerpc/mm/slice.c: At top level:
arch/powerpc/mm/slice.c:682:15: error: no previous prototype for 'arch_get_unmapped_area' [-Werror=missing-prototypes]
 unsigned long arch_get_unmapped_area(struct file *filp,
               ^
arch/powerpc/mm/slice.c:692:15: error: no previous prototype for 'arch_get_unmapped_area_topdown' [-Werror=missing-prototypes]
 unsigned long arch_get_unmapped_area_topdown(struct file *filp,
               ^

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>