The ornamental design for a headboard, as shown and described.
FIG. 1 is a front elevational view of a headboard, showing my new design;
FIG. 2 is a front, left side perspective view of the design, and the front, right side perspective view is a mirror image;
FIG. 3 is a partial cross-sectional view at A-A in FIG. 1; and,
FIG. 4 is a right side view of the design, and the left side view is the mirror image;
The broken lines are for environmental purposes only and form no part of the claimed design.