The ornamental design for a footstool, as shown and described.
FIG. 1 is a front perspective view of a foot stool;
FIG. 2 is a front elevation view of FIG. 1;
FIG. 3 is a rear elevation view of FIG. 1;
FIG. 4 is a left side elevation view of FIG. 1;
FIG. 5 is a right side elevation view of FIG. 1;
FIG. 6 is a top plan view of FIG. 1;
FIG. 7 is a bottom plan view of FIG. 1; and,
FIG. 8 is a front perspective view of the foot stool, shown in use.
The evenly-sized broken lines shown only in FIG. 8 depict environmental subject matter that forms no part of the claimed design, while the remaining evenly-sized broken lines shown in the drawings depict portions of the foot stool that form no part of the claimed design, and the dash-dot-dash lines unevenly-sized broken lines represent the bounds of the claimed design.