We claim the ornamental design for a luminaire, as shown and described.
FIG. 1 is a bottom-right front perspective view of a luminaire showing our new design;
FIG. 2 is a top-front perspective view thereof;
FIG. 3 is bottom-right side perspective view thereof;
FIG. 4 is a bottom-rear perspective view thereof; and,
FIG. 5 is top-left rear perspective view thereof.
The broken line showing of portions of the luminaire depicts environment and forms no part of the claim; the broken lines immediately adjacent surfaces shown in solid lines depict boundaries of the claim.