I claim the ornamental design for a linkage member for an exercise device, as shown and described.
FIG. 1 is a front side perspective view of a linkage member for an exercise device;
FIG. 2 is a rear side perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 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.