The ornamental design for a shower enclosure, as shown and described.
FIG. 1 is a perspective view of a shower enclosure of our design;
FIG. 2 is a right side view of the shower enclosure of FIG. 1; and,
FIG. 3 is a front view of the shower enclosure of FIG. 1.
The broken lines showing structural supports and peripheral components in FIGS. 1-3 are for illustrative purposes only and form no part of the claimed design. For the embodiment, the left side view, the rear view, the top view, and the bottom view are plain and unornamented and for no part of the claimed design.