QtWebSockets: Add missing qt.branch property

This should be defined per module and branch, otherwise it always
defaults to 'stable'.

Change-Id: Ia84e051a350c8df891186618faab8e40d6152f7b
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>