filesource: Make the uri property CONSTRUCT_ONLY
filesource: Make the uri property CONSTRUCT_ONLY

This is the way it should always have been.

cfoch/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/gst-editing-services,cfoch/ges,pitivi/gst-editing-services,pitivi/gst-editing-services,pitivi/gst-editing-services