reproducible builds: make qml dependency explicit

Arnout Engelenrequested to merge
raboof/libplasma:rb-explicit-dependencies-rebased-on-master into master

Similar to https://qt-project.atlassian.net/browse/QTBUG-137440

To fix https://bugs.kde.org/show_bug.cgi?id=512868

I'll admit I don't quite know what I'm doing here, I'm mostly guessing and mimicking, but it does seem to remove the nondeterminism :)

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