The ornamental design for a frontgrill for an automobile, as shown and described.
FIG. 1 is a perspective view of a front grill for an automobile;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof, and,
FIG. 4 is a right side view thereof.
The broken lines in the drawings illustrate portions of the front grill for an automobile that are environmental structures and form no part of the claimed design.