The ornamental design for buckle, as shown and described.
FIG. 1 is a front view of a buckle showing of my new design, wherein the broken lines are included for the purpose of illustrating unclaimed portions of the buckle and form no part of the claimed design; and,
FIG. 2 is a left perspective view of FIG. 1, wherein the broken lines are included for the purpose of illustrating unclaimed portions of the buckle and form no part of the claimed design.