The ornamental design for footwear sole, as shown and described.
FIG. 1 is a perspective view of the footwear sole embodying the new design;
FIG. 2 is a top view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 is a rear view thereof.
FIGS. 1-6 show features of an article of footwear in phantom lines. These features are shown for environmental purposes and do not form any part of the claimed design. Further, the phantom lines immediately adjacent the solid-line portions of the design form the boundary of the design, with the phantom lines forming no part of the claimed design.