Add icon to KBuildSycocaProgressDialog cancel button

Summary: The cancel button in the KBuildSycocaProgressDialog (showing that the system configuration is updated) has no icon, so this fixes it.

Test Plan: Open KMenuEdit and click save.

Reviewers: ngraham, Frameworks, cfeck, elvisangelaccio

Reviewed By: ngraham, elvisangelaccio

Subscribers: bcooksley, elvisangelaccio, ngraham, kde-frameworks-devel

Tags: VDG, Frameworks

Differential Revision: https://phabricator.kde.org/D17281