kbuild: Fix path to scripts/setlocalversion

Commit 0a564b26

 broke LOCALVERSION for O=... builds. Ouch.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-and-tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Peter Anvin <hpa@zytor.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>