Fix QIcon button styling from qrc:/ with KDE Plasma style

Another QQC2 style fix; this is going to land eventually in KF5 5.76,
but applies to 5.74 (current ports version) and 5.75 (upcoming).

Reported upstream as and obtained from
https://bugs.kde.org/show_bug.cgi?id=427449
https://invent.kde.org/frameworks/qqc2-desktop-style/-/merge_requests/35

The symptoms are toolbar icons in QML-based applications would not
be rendered (at all), while switching styles to Qt-internal styles
or disabling styling entirely would render the icons; this was
spotted by swills@ while packaging Redis Desktop Manager.

Reported by: swills
Obtained from: KDE