The ornamental design for headstock for guitar, as shown and described.
FIG. 1 is a front elevation view of a shape of headstock for guitar, showing my design.
FIG. 2 is a front elevation view thereof in an exemplary environment
FIG. 3 is a back elevation view thereof.
FIG. 4 is a left side elevation view thereof; and,
FIG. 5 is a right elevation view thereof.
The broken lines showing portions of the headstock for guitar are included for the purpose of illustrating the environmental structure and form no part of the claimed design.