The ornamental design for a buckle, as shown and described.
FIG. 1 is a front view of a buckle showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a front view thereof, showing how the buckle preferably can be installed with strapping;
FIG. 8 is a rear view thereof, showing how the buckle preferably can be installed with strapping;
FIG. 9 is a front perspective view thereof; and,
FIG. 10 is a rear perspective view thereof.
The broken lines showing a recessed area in FIGS. 4, 8 and 10 and strapping in FIGS. 7 and 8 is for illustrative purposes only and forms no part of the claimed design.