m68knommu: include asm-generic/pgtable.h

Include asm-generic/pgtable.h to pick up the lazy_mmu_mode and
lazy_cpu_mode macros. Won't build without them.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>