The ornamental design for a tailgate for an automobile, as shown and described.
FIG. 1 is a perspective view of right, rear, and top of a tailgate for an automobile showing our new design;
FIG. 2 is a perspective view of left, rear, and top of thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top view thereof.
The portions of the tailgate for an automobile shown throughout the drawing views in a deemphasized and faded out half tone form are for environmental and illustrative purposes and form no part of the claimed design.