kbuild: Fix bug in make deb-pkg when using seperate source and output directories

From: Ryan Anderson <ryan@michonline.com>

When running "make O=something deb-pkg", I get a failure that claims I
haven't configured my kernel (I have).  Running it a second time tells
me to run "make mrproper"  (include/linux/version.h got built on the
first run)

Original patch from:
From: Ajay Patel <patela@gmail.com>

With modifications from:
Signed-off-By: Ryan Anderson <ryan@michonline.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>