I claim the ornamental design for a wobble stool, as shown and described.
FIG. 1 is a top, front, right isometric view of my new design for a wobble stool;
FIG. 2 is a bottom, rear, left isometric view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The evenly dashed broken lines in the drawings depict portions of the wobble stool that form no part of the claimed design. The unevenly dashed broken lines depict the boundaries of the claim and form no part of the claimed design.