The ornamental design for belt buckle, as shown and described.
1. Belt buckle
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Left
1.6 : Top
1.7 : Bottom
The broken line showing of a belt is for the purpose of illustrating environment and forms no part of the claimed design. The dot dash lines show the boundaries of the unclaimed environment and also form no part of the claimed design.