The ornamental design for a frontgrille for an automobile, as shown and described.
FIG. 1 is a first perspective view of a front grille for an automobile, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a right side elevational view thereof, the left side elevational view being a mirror image thereof.
The broken lines shown in the drawings illustrate portions of the front grille for an automobile that are environmental structures and form no part of the claimed design. The drawings include grayscale surface shading which illustrates contour, not any particular color.