powerpc/mm: Include __find_linux_pte() prototype

It fixes this W=1 compile error :

../arch/powerpc/mm/pgtable.c:337:8: error: no previous prototype for ‘__find_linux_pte’ [-Werror=missing-prototypes]
  337 | pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
      |        ^~~~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-2-clg@kaod.org