I claim the ornamental design for a lighting fixture, as shown and described.
The single view is of a light fixture showing my new design, with the fixture having five-way (72°) overall rotational symmetry.
The broken lines immediately adjacent the shaded 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.