Change license header of uitools to LGPL

uitools is a static library that user applications can link
to, and should be under LGPLv3.

Change-Id: I3a08162c0d029fc52cf13432907e297063d7a6ae
Reviewed-by: Jani Heikkinen <jani.heikkinen@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>