The ornamental design for a grill for an automobile, as shown and described.
FIG. 1 is a perspective view of a grill for an automobile showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a right side elevation view thereof.
The de-emphasized and halftone portions shown in the drawings illustrate environmental structure of a grill for an automobile that form no part of claimed design.