The ornamental design for a skidplate, as shown and described.
FIG. 1 is a perspective view of a skid plate.
FIG. 2 is a front view of the skid plate of FIG. 1.
FIG. 3 is a back view of the skid plate of FIG. 1.
FIG. 4 is a side view of the skid plate of FIG. 1.
FIG. 5 is another side view of the skid plate of FIG. 1.
FIG. 6 is a top view of the skid plate of FIG. 1; and,
FIG. 7 is a bottom view of the skid plate of FIG. 1.
The surface shadings are for illustrative purposes only, and form no part of the claimed design. The broken lines depict portions of the skid plate that form no part of the claimed design.