The ornamental design for a heel for footwear, as shown and described.
FIG. 1 is a bottom perspective view of a heel for footwear, showing our new design;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a rear elevational view thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines shown in the figures are included for the purpose of illustrating the environment in which the design is used and form no part of the claimed design.