The ornamental design for a skateboard, as shown and described.
FIG. 1 is a perspective view of a skateboard showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left view thereof;
FIG. 5 is a right view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is another perspective view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claim and all other broken lines represent portions of the skateboard that form no part of the claimed design. None of the broken lines form a part of the claim.