I claim the ornamental design for a lighting fixture, as shown and described.
The single FIGURE is a perspective front view of a lighting fixture, for which a perspective view rotated 120° with respect thereto would be identical, and for which a perspective view rotated 240° with respect thereto would be identical.
The broken lines immediately adjacent the solid-lined areas represent the bounds of the claimed design, while all other broken lines (e.g., shades and shade supports) are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.