Avoid a QStringRef -> QString conversion by using the new qt_hash overload

During qHash refactorings, this line was changed as qt_hash didn't have
an overload taking a QStringRef. This causes a performance regression
w.r.t. the same code in Qt 4.

Task-number: QTBUG-30821
Change-Id: I17b27a54a73cb9061c20f1bd7f79d0c405050edd
Reviewed-by: hjk <hjk121@nokiamail.com>