The ornamental design for a diffuser, as shown and described.
FIG. 1 is a perspective view of the diffuser embodying the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
FIGS. 2-5 and 7 illustrate unclaimed portions of the diffuser in broken lines. The broken line portions of the diffuser form no part of the claimed design.