The ornamental design for a backrest, as shown and described.
FIG. 1 is a front isometric view of a backrest showing my new design;
FIG. 2 is a rear isometric view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 a top view thereof;
FIG. 7 is a left side view thereof; and,
FIG. 8 is a right side view thereof.
The design is to be applied to a backrest and the novelty resides in the features of shape and/or configuration of the backrest as shown in the accompanying representations.