Fix build error on BlackBerry10

The variable type was changed to QStringRef but the
the conversion to QStringRef was left out.

Change-Id: I973b0eb5795fa7bcf89ceac809fb3ff607a85913
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>