The ornamental design for a buckle, as shown and described.
FIG. 1 is a top, rear, right side perspective view of a buckle showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof; and,
FIG. 5 is a top, front, left perspective view thereof.
The dash-dash broken lines represent portions of the buckle that form no part of the claimed design. The dot-dash broken lines represent boundaries of the claimed design and form no part of the claimed design.