Add libxkbcommon to 3rd party libs

This library is required by the XCB platform plugin. As we depend on
very recent version of this library and it might not be available
in base repositories of distributions, users can use -qt-xkbcommom
switch to build Qt with the bundled version.

Change-Id: I0ed2a5cc2f1df98b0e7cc926cabfa69818674e08
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>