The ornamental design for an incense burner, as shown and described.
FIG. 1 is a perspective view of the incense burner.
FIG. 2 is a front elevational view of the incense burner with the back elevation and the two side elevations being mirror images of the same.
FIG. 3 is a top plan view of the incense burner; and,
FIG. 4 is a bottom plan view of the incense burner.
The bottom of the incense burner, defined as that surface inwardly of the broken lines in FIG. 4, forms no part of the claimed design. The broken lines form no part of the claimed design.