kbuild: rpm-pkg: introduce a simple changelog section for kernel.spec

Fix the following rpmbuild warning:

  $ make srcrpm-pkg
  ...
  RPM build warnings:
      source_date_epoch_from_changelog set but %changelog is missing

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>