um: Fix out-of-tree build

Commit 30b11ee9a (um: Remove copy&paste code from init.h)
uncovered an issue wrt. out-of-tree builds.
For out-of-tree builds, we must not rely on relative paths.
Before 30b11ee9a it worked by chance as no host code included
generated header files.

Change-Id: I15e55a3db98d51a6340a440a5d07c63c334bd50f
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 0b5aedfe0e6654ec54f35109e1929a1cf7fc4cdd
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Srinivasarao P <spathi@codeaurora.org>