The ornamental design for a buckle, as shown and described.
1. Buckle
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Left
1.6 : Top
1.7 : Bottom
The buckle of the present design is shown in greyscale, as protection of the present design is not to be limited to a specific colour and/or combination of colours of the buckle and of its parts.