Fix compilation on Windows.
Fix compilation on Windows.

Apparently, the friend declaration needs to have the linkage specified
as well.

Change-Id: Ied1d8c6a69688d82dba041ec42cd2ede4da5a55e
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

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