The ornamental design for a headstock, as shown and described.
FIG. 1 is a first perspective view of a headstock showing my new design.
FIG. 2 is a top plan view thereof.
FIG. 3 is a left side elevational view thereof.
FIG. 4 is a right side elevational view thereof.
FIG. 5 is a front elevational view thereof.
FIG. 6 is a rear elevational view thereof.
FIG. 7 is an enlarged view of the headstock as shown in FIG. 1; and,
FIG. 8 is an enlarged view of the headstock as shown in FIG. 5.
The broken lines in FIGS. 1-8 showing portions of a guitar body and neck are included for the purpose of illustrating environmental structure and form no part of the claimed design.