Doc: Adding "qtdoc" to the depends qdocconf variable.

Needed in order to link to the main Qt 5.2 documentation.

Change-Id: I451f4d60f165befe179cfe58904876c42211cfee
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>