kbuild: fix "cat: .version: No such file or directory"

Since commit 2df8220c ("kbuild: build init/built-in.a just once"),
the .version file is not touched at all when KBUILD_BUILD_VERSION is
given.

If KBUILD_BUILD_VERSION is specified and the .version file is missing
(for example right after 'make mrproper'), "No such file or director"
is shown. Even if the .version exists, it is irrelevant to the version
of the current build.

  $ make -j$(nproc) KBUILD_BUILD_VERSION=100 mrproper defconfig all
    [ snip ]
    BUILD   arch/x86/boot/bzImage
  cat: .version: No such file or directory
  Kernel: arch/x86/boot/bzImage is ready  (#)

Show KBUILD_BUILD_VERSION if it is given.

Fixes: 2df8220c ("kbuild: build init/built-in.a just once")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>