The ornamental design for a frontgrill for an automobile, as shown and described herein.
FIG. 1 is a perspective view of a front grill for an automobile, showing our 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 broken lines in the drawings illustrate portions of the front grill for an automobile that are environmental and form no part of the claimed design.