The ornamental design for a windturbine, as shown and described.
FIG. 1 is a top perspective view of a wind turbine showing the new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a top perspective view thereof in an alternate, open configuration.
The claimed design of the wind turbine is symmetrical. The left side, right side, and rear view are the same as shown in FIG. 2, and the bottom plan view is the same as shown in FIG. 3.