fix documentation target
fix documentation target

We must pass QT_INSTALL_DOCS to qdoc, because qdoc doesn't know about
it in Qt 5.1.x.

Change-Id: Iccf5fc7bc194b0c5c39f0a8464977d1b85a4709d
Reviewed-by: Jerome Pasion <c10995d24f6b454503389e8d00d837aef038985d@digia.com>

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