[PATCH] uml: add a build dependency

kern_constants.h now depends on arch/um/include to make sure it exists
before we try to create symlinks in it.

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