The ornamental design for an headlight accessory, as shown and described herein.
FIG. 1 is a perspective view of the headlight accessory in accordance with my invention;
FIG. 2 is a side view thereof, taken from the left side of FIG. 1;
FIG. 3 is a side view thereof, taken from the right side of FIG. 1;
FIG. 4 is a front end view thereof;
FIG. 5 is a base end view thereof;
FIG. 6 is a top view thereof; and
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view showing a pair of headlight accessories installed on a vehicle.
Broken lines shown in FIG. 8 are for illustrative purposes only and form no part of the claimed design. The surface indicia of parallel lines also form no part of the claimed design and are merely surface shading.