The ornamental design for a headrest, as shown and described.
FIG. 1 is a first perspective view of a headrest.
FIG. 2 is a front view of the headrest.
FIG. 3 is a back view of the headrest.
FIG. 4 is a left side view of the headrest.
FIG. 5 is a right side view of the headrest.
FIG. 6 is a top view of the headrest.
FIG. 7 is a bottom view of the headrest; and,
FIG. 8 is the first perspective view of the headrest coupled to a vehicle seat.
The scope of this disclosure extends to the entirety of the headrest illustrated in the figures. The dashed lines shown represent environmental structure or boundaries that do not form part of the claimed design.