The ornamental design for a skateboard, as shown and described.
FIG. 1 is a top perspective view of a skateboard of my new design;
FIG. 2 is a bottom perspective view of the skateboard;
FIG. 3 is a right side view of the skateboard, the left side being a mirror image;
FIG. 4 is a front view of the skateboard;
FIG. 5 is a rear view of the skateboard;
FIG. 6 is a top view of the skateboard;
FIG. 7 is a bottom view of the skateboard;
FIG. 8 is a cross-sectional view of the skateboard taken along line 8-8 in FIG. 3; and,
FIG. 9 is an enlarged, partial right, rear side view of the skateboard taken at area 8-8 in FIG. 3, the left side being a mirror image.
The dash-dot broken lines represent the bounds of the claim and all other broken lines represent unclaimed portions of the design. None of the broken lines form a part of the claimed design.