Fix blurry icons by adding UseHighDpiPixmaps flag

Summary: When using fractional scaling, some icons are blurred. This patch fixes the issue.

Test Plan:
No side effects.

Before:
{F8287226}
After:
{F8287229}

Reviewers: #kde_applications, ngraham

Reviewed By: #kde_applications, ngraham

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D29452