The ornamental design for a kickboard, as shown and described.
FIG. 1 is a perspective view of a kickboard showing my new design;
FIG. 2 is a front elevational view of the kickboard shown in FIG. 1;
FIG. 3 is a left side elevational view of the kickboard shown in FIG. 1, the right side elevational view is a mirror image of the left side elevational view;
FIG. 4 is a top plan view of the kickboard shown in FIG. 1, the bottom plan view is a mirror image of the top plan view; and,
FIG. 5 is a rear elevational view of the kickboard shown in FIG. 1.