The ornamental design for a front grille for an automobile, as shown and described.
FIG. 1 is a front perspective view of a front grille for an automobile showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a right-side view thereof, with the left-side view being a mirror image.