I claim the ornamental design for a lighting fixture, as shown and described.
FIG. 1 is a perspective view of a lighting fixture having mirror image symmetry (one hundred eighty degree overall rotational symmetry, and mirror image symmetry along a first vertical imaginary plane and also mirror image symmetry along a second vertical imaginary plane, with the first and second vertical imaginary planes being orthogonal to each other);
FIG. 2 is a top plan view of the lighting fixture of FIG. 1; and,
FIG. 3 is a bottom plan view of the lighting fixture of FIG. 1.
The broken lines shown on the drawing are for illustrative purposes only and form no part of the claimed design.