Linux: Use kbuild to check for UML

Linux: Use kbuild to check for UML

<linux/autoconf.h> is automatically included by kbuild; you aren’t
supposed to include it directly.  That breaks in 2.6.33 because
<linux/autoconf.h> is moving to <generated/autoconf.h>.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Change-Id: I56c90dfe90ef72408620793fcd4e927ea3d8f468
Reviewed-on: http://gerrit.openafs.org/1350
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 660c7a3952f36fc892e12aa6b11eda3295b1416d)
Reviewed-on: http://gerrit.openafs.org/1374