winrt: MessageDialogs signals not fired.

Steps:
1. Run system dialog's example on windows phone
2. Select Message tab.
3. Press open.
4. Dismiss dialog
Expected: System output to print "qml: clicked button 1024" and visibility of the dialog set to false

Actual: No print, dialog remains visible and system out prints:
QObject::connect: Cannot queue arguments of type 'QPlatformDialogHelper::StandardButton'
(Make sure 'QPlatformDialogHelper::StandardButton' is registered using qRegisterMetaType().)