The ornamental design for a skateboardrack, as shown and described.
FIG. 1 is a top, front, and left side perspective view of a skateboard rack showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view of thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is an additional top, front, and left side perspective view thereof, with the broken line disclosure of skateboards included for the purpose of showing environmental structure and forming no part of the claimed design.