UML - Fix build in 2.6.24-rc2-mm1

The earlier pgtable.h tidying patch made things a bit too tidy.  Add
back a header which is needed in VMALLOC_START and friend.  Also add
back a definition of pmd_page_vaddr, which is needed on x86_64.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>