Since 2df8220c ("kbuild: build init/built-in.a just once"),
generating Debian packages using 'make bindeb-pkg' results in
packages that are stuck to the same .version, leading to unexpected
behaviours (multiple packages with the same version).

That's because the mkdebian script samples the build version
before building the kernel, and forces the use of that version
number for the actual build.

Restore the previous behaviour by calling init/build-version
instead of reading the .version file. This is likely to result
in too many .version bumps, but this is what was happening before
(although the bump was affecting builds made after the current one).

Fixes: 2df8220c ("kbuild: build init/built-in.a just once")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>