The ornamental design for a set of spikes for a garden edging, as shown and described.
FIG. 1 is a perspective view of a set of spikes for a garden edging showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top view thereof.
The broken lines in the figures are for the purpose of illustrating environmental structure and form no part of the claimed design. The broken line areas within the spikes illustrate portions of the set of spikes for a garden edging that form no part of the claimed design.