The ornamental design for a luminaire, as shown and described.
FIG. 1 is a perspective view of the luminaire;
FIG. 2 is a front view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is another side view thereof.
In the drawings, the portions shown by broken lines form no part of the claimed design. Further, in the drawings, clusters of oblique lines are used to show surfaces of the luminaire that are transparent and translucent, as demonstrated, for example, by the polygonal surface in FIG. 5.