The ornamental design for a bookend, as shown and described.
FIG. 1 is a front, side and top perspective view of a bookend showing my new design; and,
FIG. 2 is a close-up view thereof.
The bottom forms no part of the claimed design.
The broken lines immediately adjacent to the claim depict the bounds of the claimed design, while all other broken lines are directed to environment. The broken lines form no part of the claimed design.