The ornamental design for a windturbine, as shown and described.
FIG. 1 is a perspective view of the wind turbine.
FIG. 2 is a side elevation view of the wind turbine rotated to a first position.
FIG. 3 is a side elevation view of the wind turbine rotated to a second position.
FIG. 4 is a side elevation view of the wind turbine rotated to a third position.
FIG. 5 is a side elevation view of the wind turbine rotated to a fourth position.
FIG. 6 is a bottom plan view of the wind turbine; and,
FIG. 7 is a top plan view of the wind turbine.
The broken line showing of bores or circles are included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.