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