libzipplugin: Add missing <memory> header

Summary:
Build fails on some setups for use of std::unique_ptr.
See also: https://bugs.gentoo.org/653546

Reviewers: #ark, elvisangelaccio

Reviewed By: #ark, elvisangelaccio

Subscribers: heikobecker, kde-utils-devel

Tags: #ark

Differential Revision: https://phabricator.kde.org/D12374