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

arch/powerpc/mm/pgtable.c

@@ -26,6 +26,7 @@
 #include <asm/tlbflush.h>
 #include <asm/tlb.h>
 #include <asm/hugetlb.h>
+#include <asm/pte-walk.h>
 
 static inline int is_exec_fault(void)
 {