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 rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof, the left side a mirror image thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing represents features of the buckle that form no part of the claimed design. The shading lines shown on the surface of the buckle are used to specify a three dimensional shape of the claimed portion of the buckle.