The ornamental design for a luminaire, as shown and described.
FIG. 1 is a perspective of a luminaire, with certain surfaces and features of the luminaire being shown in broken lines and forming no part of the claimed invention;
FIG. 2 is a front elevational view of the luminaire;
FIG. 3 is a top plan view of the luminaire;
FIG. 4 is a rear elevational view of the luminaire;
FIG. 5 is a bottom plan view of the luminaire;
FIG. 6 is a right side elevational view of the luminaire; and,
FIG. 7 is a left elevational view of the luminaire.
Features forming no part of the claimed design are illustrated in broken lines.