The ornamental design for a luminaire, as shown and described.
FIG. 1 is a left front top perspective view of a luminaire showing my new design.
FIG. 2 is a left bottom perspective view thereof, showing two tubular elements separated by a gap, the depth of said gap forming no part of the claimed design.
FIG. 3 is a rear elevational view thereof.
FIG. 4 is a left side elevational view thereof.
FIG. 5 is a front elevational view thereof.
FIG. 6 is a right side elevational view thereof.
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines in FIGS. 1, 5, and 7 form no part of the claimed design.