The ornamental design for a windturbine, as shown and described.
FIG. 1 is a front and right side perspective view of a wind turbine showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a rear and left side perspective view thereof.
The portions of the wind turbine shown in broken lines is for the purpose of illustrating environmental structure only and forms no part of the claim. Where broken lines abut a claimed surface it is understood that those broken lines represent an unclaimed boundary between claimed and unclaimed surfaces. None of the broken lines form any part of the claimed design.