kbuild: fix make O=... build

It fixes the following error:

make[1]: *** No rule to make target `include/asm', needed by `arch/alpha/kernel/asm-offsets.s'.  Stop.

Reported by:
From: Jan Dittmer <j.dittmer@portrix.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>