The ornamental design for a skateboard, as shown and described.
1. Skateboard
1.1 : Top
1.2 : Perspective
1.3 : Perspective
Skateboard deck design. 1.1) top view; 1.2) perspective (back, top, left); 1.3) perspective (front, top, right); The parts shown by means of broken lines in the reproductions are not part of the claimed design.