The ornamental design for an automobile side skirt, as shown and described.
FIG. 1 is perspective view of an automobile side skirt showing the side that faces away from the automobile, with the end that would be disposed toward the rear of the automobile when the side skirt is installed shown in the upper left hand side of the figure;
FIG. 2 is a side elevation view, showing the surface that would be visible to someone facing the side of the automobile when the side skirt is installed on an automobile;
FIG. 3 is a rear elevation view, showing the end on the left hand side of FIG. 2; and,
FIG. 4 is a front elevation view, showing the end on the right hand side of FIG. 2.
The rear view of the surface that faces the automobile when installed is omitted and is not part of the claimed design.