I claim the ornamental design for a panel, as shown and described.
FIG. 1 is an enlarged top view of a portion of a panel;
FIG. 2 is a top and side perspective view of the panel; and,
FIG. 3 is an enlarged top and front perspective view of a panel.
Any subject matter shown in dashed lines, and any surfaces of the design not shown, including interior surfaces, form no part of the claimed invention.