Use setIconProvider instead of setPixmapProvider for KHistoryComboBox

Summary: setPixmapProvider will be deprecated with D25700

Test Plan: Open open folder dialog, history combobox still has icons

Reviewers: #plasma, #frameworks, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T12310

Differential Revision: https://phabricator.kde.org/D25701