The ornamental design for a luminaire, as shown and described.
FIG. 1 is a perspective view of a luminaire showing the design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a left side thereof.
The symbolic break lines shown in the drawings indicate that the appearance of any portion of the article between the break lines forms no part of the claimed design. The portions of the luminaire shown in broken lines form no part of the claimed design.