The ornamental design for a frontgrille for an automobile, as shown and described.
FIG. 1 is a front and right side perspective view of a front grille for an automobile showing our new design;
FIG. 2 is a front and left side perspective view thereof;
FIG. 3 is a front elevational view;
FIG. 4 is a top plan view thereof;
FIG. 5 is a right side elevational view thereof; and,
FIG. 6 is a left side elevational view thereof.
The broken line showing of the front grille for an automobile is for purpose of illustrating environmental structure and forms no part of the claimed design. The rear view and the bottom view of the front grille for an automobile form no part of the claimed design.