Migrate to ecm_qt_declare_logging_category

Summary:
During D12656, it was suggested that it would be better to migrate to
ecm_qt_declare_logging_category.

Test Plan: It builds

Reviewers: #plasma, ivan, lbeltrame, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12658