The ornamental design for a headlight for a motorvehicle, as shown and described.
FIG. 1 is a first perspective view of a headlight for a motor vehicle;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a top view thereof.
The broken lines depict portions of the headlight for a motor vehicle in which the design is embodied that are not considered part of the claimed design.