Doc: Enabled linking to Qt WebChannel.

Added the project to the depends variable
and added it to the list of modules.

Change-Id: I80ad2bada0bf28489c080cbc5a373103646c3b5e
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>