The ornamental design for a skateboard, as shown and described.
FIG. 1 is a top, front and right-side perspective view of a skateboard embodying our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right-side elevational view thereof;
FIG. 5 is a left-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown are included for the purpose of illustrating the environment in which the article resides and forms no part of the claimed design.