Make target_link_libraries for kdeinit_dolphin PRIVATE

Summary:
There is no need to add all of kdeinit_dolphin's
dependencies (including the static archive) when
linking dolphin; kdemain is the only needed symbol.
Mark the link libraries PRIVATE to simplify the
link command for dolphin.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: asturmlechner, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D12931