Find KF5I18n in the top-level CMakeLists.txt

Summary:
I'm not entirely sure why ki18n_install() (added on tarball
generation) is even known at that moment, but since 3e44fb7 in ki18n
I get "CMake Error: No script specified for argument -P" when
generating ts files, apparently because
_ki18n_build_{ts,po}files_script end up empty.
In any case finding stuff before trying to use it seems sensible
and adding the find_package call before ki18n_install indeed fixes
the problem.

Test Plan: Tested with ki18n-5.48.0 and the kde-dev-util tarball

Reviewers: #kde_applications, ltoscano

Differential Revision: https://phabricator.kde.org/D14066