change marker for drag_circle mode

[dev-aritra]

With reference to https://github.com/iamanvesh/mapbox-gl-draw-circle
There are two modes draw_circle and drag_circle.
So the marker for draw_circle is set to ADD on mouse move, hence the cursor is shown accordingly.
But there's no such thing for drag_circle mode.
Hence adding the condition for the drag_circle mode so that the cursor is set accordingly.