The ornamental design for a sidingpanel, as is shown and described.
FIG. 1 is a top front right perspective view of the siding panel according to the present design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The siding panel is indefinite length, with a textured surface of variable width. The pattern shown on the textured surface, repeats throughout the length and width of the siding panel.