The ornamental design for a guitarheadstock, as shown and described.
FIG. 1 is a perspective view of the guitar headstock showing my new design;
FIG. 2 is a front view thereof:
FIG. 3 is a rear view thereof;
FIG. 4 is left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top view thereof.
Throughout the views, the bottommost broken line of the guitar headstock represents the boundary of the claimed design. The remaining broken lines represent the portions of the design, which form no part of the claim.