I claim an ornamental design for a buckle, as shown and described.
FIG. 1 is a front elevational view of the claimed design.
FIG. 2 is a top view of the claimed design.
FIG. 3 is a left side elevational view of the claimed design.
FIG. 4 is a right side elevational view of the claimed design.
FIG. 5 is a rear elevational view of the claimed design; and,
FIG. 6 is a bottom view of the claimed design.
The broken line showing of elements in FIGS. 2-6 is included for the purpose of illustrating environmental matter and forms no part of the claimed design.