The ornamental design for a diffuser, as shown and described.
FIG. 1 is a first perspective view of a diffuser showing my design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a bottom view thereof; and,
FIG. 9 is an enlarged view of the selected portion of FIG. 2.
In the drawings, the dashed broken lines depict portions of the diffuser that form no part of the claim. The dot-dash broken lines in FIGS. 2 and 9 represent the boundary of the enlarged area of the claimed design and form no part of the claim.