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

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>

arch/powerpc/mm/dump_linuxpagetables.c

@@ -19,6 +19,7 @@
 #include <linux/hugetlb.h>
 #include <linux/io.h>
 #include <linux/mm.h>
+#include <linux/highmem.h>
 #include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <asm/fixmap.h>