I claim the ornamental design for a linkage for an exercise device, as shown and described.
FIG. 1 is a rear side perspective view of a linkage for an exercise device;
FIG. 2 is a top view thereof;
FIG. 3 is a first side view thereof;
FIG. 4 is a second side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a bottom view thereof.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken (dashed) lines shown in the drawings are included for the purpose of illustrating environmental elements only and form no part of the claimed design.