i386: Fix leak of ../kernel from top level

Fix a compile error when the directory above the kernel source contains
a file named "kernel".  Originally from Ben LaHaise, modified based on
feedback from Sam Ravnborg

Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Ben LaHaise <bcrl@kvack.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>