I claim the ornamental design for a headboard, as shown and described.
FIG. 1 is a front elevation view of my new design;
FIG. 2 is a left side view thereof, the right side view being a mirror image thereof; and,
FIG. 3 is a front perspective view thereof.
The views of the headboard not shown form no part of the claimed design. The dash-dash lines in the figures represent features and environmental elements that form no part of the claimed design. The dash-dot line in each figure represents the unclaimed boundary of the claimed design.