The ornamental design of a lighthead, as shown and described.
FIG. 1 is a perspective view of a light head;
FIG. 2 is a top view of the light head;
FIG. 3 is a front view of the light head, the opposite rear view being identical;
FIG. 4 is a side view of the light head, the opposite side view being identical; and,
FIG. 5 is the bottom view of the light head.
The broken line showing of the mounting bracket and the screws is for the purpose of illustrating environmental structure for the light head and forms no part of the claimed design.