The ornamental design for a buckle, as shown and described.
FIG. 1 is a perspective view of the front, bottom and right side of a buckle, showing our new design, in which a plug and a socket are engaged with each other;
FIG. 2 is a front view of the buckle;
FIG. 3 is a rear view of the buckle;
FIG. 4 is a right side view of the buckle, the left side view being mirror image thereof;
FIG. 5 is a top plan view of the buckle;
FIG. 6 is a bottom view of the buckle;
FIG. 7 is a perspective view of the front, bottom and right side of the plug disengaged from the socket;
FIG. 8 is a front view of the plug; and,
FIG. 9 is a rear view of the plug.
The broken lines depict portions of the buckle that form no part of the claimed design.