The ornamental design for a luminaire, as shown and described.
FIG. 1 is a bottom perspective view of a luminaire showing our new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a right side elevational view thereof, the left, front, and rear elevations being identical thereto;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a top plan view thereof.
The broken lines shown in FIGS. 1-3 and 5 represent a pendent structure and cover plates used in mounting the luminaire to a ceiling. The broken line structures form no part of the claimed design.