kbuild: rpm-pkg: make sure to have versioned 'Obsoletes' for kernel.spec

Fix the following rpmbuild warning:

  $ make srcrpm-pkg
  ...
  RPM build warnings:
      line 34: It's not recommended to have unversioned Obsoletes: Obsoletes: kernel-headers

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