The ornamental design for platform sole for footwear, as shown and described.
FIG. 1 is a front perspective view of a platform sole showing my footwear design;
FIG. 2 is a left side view of the platform sole shown in FIG. 1;
FIG. 3 is a right side view of the platform sole shown in FIG. 1;
FIG. 4 is a front view of the platform sole shown in FIG. 1;
FIG. 5 is a rear view of the platform sole shown in FIG. 1; and,
FIG. 6 is a top view of the platform sole shown in FIG. 1.
The broken line showing of the sole, heel, heel cap, body of shoe and footbed is for illustrative purposes only and forms no part of the claimed design.