The ornamental design for a buckle, as shown and described.
FIG. 1 is a perspective view of a buckle according to the present invention.
FIG. 2 is a front view of the invention shown in FIG. 1.
FIG. 3 is a rear view of the invention shown in FIG. 1.
FIG. 4 is a side view of the invention shown in FIG. 1.
FIG. 5 is a second side view of the invention shown in FIG. 1.
FIG. 6 is a top view of the invention shown in FIG. 1.
FIG. 7 is a bottom view of the invention shown in FIG. 1; and,
FIG. 8 is a perspective view of the invention shown in FIG. 1 wherein the buckle is shown in an environment of use. The broken line showing of a shoe depicts environmental subject matter that forms no part of the claimed design.