Add back (accidentally?) deleted line in CMakeLists.txt

Without this, ToolButton.qml (Plasma Components 3) is not installed and
applets like the mediacontroller do not load because they can't find it.

The line has been removed in 61269548168d5cd08a60826c7d417b002a698723,
and it looks like an unrelated change.

Marco, feel free to revert if the proper solution is different.

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