The ornamental design for a cap for a fitting, as shown and described.
FIG. 1 is a side view of a cap for a fitting showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof; and,
FIG. 4 is a side view of the cap located on a fitting retained in a housing.
The fitting and housing shown by dashed lines in FIG. 4 are included to illustrate an associated environment and form no part of the claimed invention.