The ornamental design for a headboard, as shown and described.
FIG. 1 is a front elevational view of a headboard showing my new design,
the broken line showing is for illustrating environmental elements only
and forms no part of the claimed design;
FIG. 2 is a side elevational view thereof, the opposite side being a mirror
image;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is an enlarged partial front perspective, illustrating the left end
thereof.
The bottom of the article is not part of the claimed design.