The design for the decorative belt buckle, as shown and described.
FIG. 1. is a perspective view of the decorative belt buckle.
FIG. 2. is a front plan view of the decorative belt buckle.
FIG. 3. is a right-side plan view of the decorative belt buckle.
FIG. 4. is a left-side plan view of the decorative belt buckle.
FIG. 5. is a back plan view of the decorative belt buckle.
FIG. 6. is a bottom plan view of the decorative belt buckle; and,
FIG. 7. is a top plan view of the decorative belt buckle.
The broken lines in figures are included for the purpose of illustrating portions of the decorative belt buckle that form no part of the claimed design.