The ornamental design for a headboard, as shown and described.
FIG. 1 is a right front perspective view of a headboard showing our new
design;
FIG. 2 is a side view, the opposite side being a mirror image of the side
shown; and,
FIG. 3 is a front elevational view thereof.
The top, bottom and rear views are not shown and form no part of the
claimed design. The dotted lines represent environmental elements only,
and form no part of the claimed design.