kbuild: rpm-pkg: do not include depmod-generated files

Installing the kernel package is fine, but when uninstalling it, the
following warnings are shown:

  warning: file modules.symbols.bin: remove failed: No such file or directory
  warning: file modules.symbols: remove failed: No such file or directory
  warning: file modules.softdep: remove failed: No such file or directory
  warning: file modules.devname: remove failed: No such file or directory
  warning: file modules.dep.bin: remove failed: No such file or directory
  warning: file modules.dep: remove failed: No such file or directory
  warning: file modules.builtin.bin: remove failed: No such file or directory
  warning: file modules.builtin.alias.bin: remove failed: No such file or directory
  warning: file modules.alias.bin: remove failed: No such file or directory
  warning: file modules.alias: remove failed: No such file or directory

The %preun scriptlet runs 'kernel-install remove', which in turn invokes
/usr/lib/kernel/install.d/50-depmod.install to remove those files before
the actual package removal.

RPM-based distributions do not ship files generated by depmod. Mark them
as %ghost in order to exclude them from the package, but still claim the
ownership on them.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>