kbuild: version.h should depend on .kernelrelease

Rebuilding a previously built tree while using make's -j options from time to
time results in the version.h check running at the same time as the updating
of .kernelrelease, resulting in UTS_RELEASE remaining an empty string (and as
a side effect causing the entire kernel to be rebuilt).

Signed-Off-By: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>