I claim the ornamental design for a footstool, as shown and described.
FIG. 1: is a perspective view of a footstool showing my new design;
FIG. 2: is a front view thereof;
FIG. 3: is a back view thereof;
FIG. 4: is a right side view thereof;
FIG. 5: is a left side view thereof;
FIG. 6: is a top view thereof;
FIG. 7: is a bottom view thereof;
FIG. 8: is an exploded perspective view of the frame portion of the footstool without the cushion; and,
FIG. 9: is a perspective view of the cushion portion of the footstool.
The broken lines in the figures are for the purpose of illustrating portions of the footstool, which form no part of the claimed design. The footstool has been shown with the frame and cushion separately in FIGS. 8 and 9 to show aspects of the design that may not be apparent in the other views.