The ornamental design for a buckle, as shown and described.
FIG. 1 is a front perspective view of a buckle showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a front perspective view of the buckle shown in FIG. 1, shown in the environment of a handbag; and,
FIG. 8 is a front perspective view of the buckle shown in FIG. 1, shown in the environment of a belt.
The broken lines in the drawings depict portions of the buckle and environmental subject matter that form no part of the claimed design. No claim is made to the back of the design.