The ornamental design for a headset accessory, as shown and described.
FIG. 1 is a perspective view of a headset accessory showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is an exploded view of the headset accessory thereof; and,
FIG. 9 is an exploded view of the headset accessory thereof.
The broken lines shown in the drawings depict portions of the headset accessory in which the design is embodied that form no part of the claimed design.