The ornamental design for a skateboard, as shown and described.
FIG. 1 is a rear perspective view showing the skateboard;
FIG. 2 is a top view of the design of FIG. 1;
FIG. 3 is a front view of the design of FIG. 1;
FIG. 4 is a side view of the design of FIG. 1;
FIG. 5 is a rear view of the design in FIG. 1;
FIG. 6 is a bottom view of the design in FIG. 1; and,
FIG. 7 is a side perspective view of the design in FIG. 1.
Phantom lining where utilized are included for the purpose of illustrating the unclaimed portions of the article and form no part of the claimed design.