The ornamental design for a headboard, as shown and described.
FIG. 1 is a front perspective view of the headboard shown in a fully opened position;
FIG. 2 is a front view of the headboard shown in FIG. 1;
FIG. 3 is a rear view of the headboard shown in FIG. 1;
FIG. 4 is a top view of the headboard shown in FIG. 1;
FIG. 5 is a left-side view of the headboard shown in FIG. 1;
FIG. 6 is a front perspective view of the headboard shown in FIG. 1, in a partially folded position; and,
FIG. 7 is a perspective view of the headboard shown in FIG. 1, in a fully folded, closed position, with the legs and latch brackets in an alternate configuration.