The ornamental design for a lighthead, as shown and described.
FIG. 1 is a perspective view of a lighthead;
FIG. 2 is a top plan view of the lighthead;
FIG. 3 is a bottom view of the lighthead;
FIG. 4 is a front view of the lighthead, the opposite rear view being identical; and,
FIG. 5 is a right side view of the lighthead, the opposite left side view being identical.
The broken lines in the drawings illustrate portions of the lighthead or the environment related to the lighthead which form no part of the claimed design.