Add missing member initialization in BuildOptions class.
Add missing member initialization in BuildOptions class.

The member specifying whether to check timestamps of generated artifacts
had a random value in a default-constructed object, which lead to
"interesting" behavior.

Change-Id: Id12a94832451bbe18791139687ea10d67da829c3
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

gatzka/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,villytiger/qbs,Distrotech/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,Distrotech/qbs,villytiger/qbs