The ornamental design for headstock portion of a guitar, as shown and described.
FIG. 1 is a perspective view of headstock portion of a guitar;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
Features shown in dashed broken lines depict portions of the headstock portion of a guitar that form no part of the claimed design. The dash-dot-dash lines shown in the drawings represent the boundaries of the claim and form no part of the claimed design.