The ornamental design for a headlight for a motorcycle, as shown and described.
FIG. 1 is a front elevational view of a head light for a motorcycle showing my new design;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof; and,
FIG. 4 is a top plan view thereof.
The broken line showing of the environment is for illustrative purposes only and forms no part of the claimed designs. The portion encompassed by a solid line is the design sought to be patented.