kbuild: rpm-pkg: ghost modules.weakdep file

In the same way as for other similar files, mark as ghost the new file
generated by depmod for configured weak dependencies for modules,
modules.weakdep, so that although it is not included in the package,
claim the ownership on it.

Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>