kbuild: unconditionally clobber include/linux/version.h on distclean

As of v3.7, the UAPI changes relocated headers around such that the
kernel version header lived in a new place.

If a person is bisecting and if you go back to pre-UAPI days,
you will create an include/linux/version.h  -- then if you checkout a
post-UAPI kernel, and even run "make distclean" it still won't delete
that old version file.  So you get a situation like this:

$ grep -R LINUX_VERSION_CODE include/
include/generated/uapi/linux/version.h:#define LINUX_VERSION_CODE 200192
include/linux/version.h:#define LINUX_VERSION_CODE 132646

The value in that second line is representative of a v2.6.38 version.
And it will be sourced/used, hence leading to strange behaviours, such
as drivers/staging content (which typically hasn't been purged of version
ifdefs) failing to build.

Since it is a subtle mode of failure, lets always clobber the old
file when doing a distclean.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>