We claim the ornamental design for a plumbing fixture, as shown and described.
FIG. 1 is a top perspective view of another embodiment of the claimed design;
FIG. 2 is a front view of the claimed design of FIG. 1;
FIG. 3 is a top plan view of the claimed design of FIG. 1; and,
FIG. 4 is a left-side view of the claimed design of FIG. 1.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.