The ornamental design for a headrest, as shown and described.
FIG. 1 is a right side perspective view of a headrest showing our new design;
FIG. 2 is a back perspective view of a headrest showing our new design;
FIG. 3 is a front view of a headrest showing our new design;
FIG. 4 is a back view of the headrest showing our new design;
FIG. 5 is a right side view of a headrest showing our new design;
FIG. 6 is a top side view of a headrest showing our new design in a first position; and,
FIG. 7 is a top side view of a headrest showing our new design in a second position.
The light, phantom, small-dashed lines located on the middle portion of the back side of the headrest of our new design shown in the Figures are for illustrative purposes only and form no part of the claimed design.