The ornamental design for a footboard, as shown and described.
FIG. 1 is a perspective view taken from a top, front, right side of a foot board, according to our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a front elevation view thereof; and,
FIG. 7 is a bottom plan view thereof.
Throughout the figures, the dashed broken lines show unclaimed structure of the foot board, and form no part of the claimed design. The dash-dot broken lines represent the boundary between claimed and unclaimed areas and form no part of the claimed design.