Fix documentation build with qbs.
Fix documentation build with qbs.

Apparently, the qdoc in Qt 5.1 no longer allows a relative path as
the source directory.

Change-Id: Ia4de7a070020ae1951affa5653dbf73475d1fed3
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

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