I claim the ornamental design for a plumbing fixture, as shown and described.
FIG. 1 is a left, top, front perspective view of a plumbing fixture embodying the new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a rear elevational view thereof; and,
FIG. 7 is a bottom plan view thereof.
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. The dot-dash lines in the drawings define boundaries of the claimed design and form no part thereof.