GenericGCC.qbs: use --sysroot option in pch compiler
GenericGCC.qbs: use --sysroot option in pch compiler

Change-Id: I31816d555da94506766235cfb9d603abd9d16d90
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@nokia.com>

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