Inject summit de.po directly in Kleopatra tarball

* packages/gen-tarball.sh: Use summit/de.po as a basis.

Ideally it should only have more strings and that is also the
file we update. The old code where we built the l10n package
apart from the package tarballs also preferred summit po files
over the stable branches.

• GnuPG-Bug-Id: T6821