The ornamental design for a windturbine, as shown and described.
FIG. 1 is a front view of the wind turbine;
FIG. 2 is a left side view of the wind turbine;
FIG. 3 is a rear view of the wind turbine;
FIG. 4 is a right side view the wind turbine;
FIG. 5 is a top view of the wind turbine;
FIG. 6 is a bottom view of the wind turbine;
FIG. 7 is a top front left perspective view of the wind turbine; and,
FIG. 8 is a top rear left perspective view of the wind turbine.
The broken lines shown in the drawings represent portions of the wind turbine that form no part of the claimed design. Where broken lines adjoin a claimed surface it is understood that those broken lines represent an unclaimed boundary between claimed and unclaimed surfaces and form no part of the claimed design.