Static builds: Let qmlimportscanner find kirigami

Install the qmldir file in static builds, too, in order for qmlimportscanner to find Kirigami. Moreover correct the plugin field in qmldir to all lowercase to match the name plugin library (not the plugin class).

I both new to static qt builds and Kirigami, so I'm not 100% certain, this is the correct fix, but Qt's own qml modules do install their qmldir in static builds.

@leinir you added the "plugin" line in qmldir. Why is it camel-case instead of all lowercase? Was that on purpose?