The ornamental design for a headrest, as shown and described.
FIG. 1 is a top-left-front perspective view of the head rest showing my design;
FIG. 2 is a bottom-rear-right perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left elevational view thereof;
FIG. 6 is a right elevational view thereof;
FIG. 7 is a front elevational view thereof; and,
FIG. 8 is a rear elevational view thereof.
The broken line showing the studs in FIGS. 1-7 and the broken line showing the slot in FIGS. 1, 2, and 8, are included for the purpose of showing environmental structure and form no part of the claimed design.