The ornamental design for a luminaire, as shown and described.
FIG. 1 is a side view of a luminaire showing my new design, where the shade has circular symmetry with the mounting portion shown in broken lines for context;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is an upper perspective view thereof with the mounting portion shown in broken lines for context; and,
FIG. 5 is a lower perspective view thereof with the mounting portion shown in broken lines for context.
The broken line showing of mounting portion is for the purpose of illustrating context and forms no part of the claimed design.