[PPC] Add linux/pagemap.h to arch/ppc/mm/tlb.c

When compiled without swap support, arch/mm/tlb.c complains about missing
function declarations. This patch fixes the warnings.

Signed-off-by: Laurent Pinchart <laurent.pinchart@technotrade.biz>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>