Fix qbs build
Fix qbs build

The plugins must not define QBS_LIBRARY.

Change-Id: Ie7fd3b5052bb37b5a25a08fa41a44e4731088761
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@qt.io>

qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs