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