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 plan 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 with other like landscape slabs.
The broken lines shown in the drawings are included for purposes of illustrating environment 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.