I claim the ornamental design for a landscape slab, as shown and described.
FIG. 1 is a perspective view of a landscape slab;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a plan view of an example condition of use showing the landscape slab illustrated in FIGS. 1-7 in combination with other landscape slabs.
The broken lines illustrate portions of the landscape slab and form no part of the claimed design. The surface shading lines show contour and/or distinguish between open and solid areas. The surface shading lines are not surface ornamentation.