powerpc/mm: Fix linux page tables build with some configs

[ Upstream commit 462951cd32e1496dc64b00051dfb777efc8ae5d8 ]

For some configs the build fails with:

  arch/powerpc/mm/dump_linuxpagetables.c: In function 'populate_markers':
  arch/powerpc/mm/dump_linuxpagetables.c:306:39: error: 'PKMAP_BASE' undeclared (first use in this function)
  arch/powerpc/mm/dump_linuxpagetables.c:314:50: error: 'LAST_PKMAP' undeclared (first use in this function)

These come from highmem.h, including that fixes the build.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>