The ornamental design for a wriststrap, as shown and described.
FIG. 1 is a perspective view of a wrist strap showing my new design;
FIG. 2 is a front elevational view thereof with the rear elevational view being identical thereto;
FIG. 3 is a right side elevation view thereof with the left side elevation view being identical thereto;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view; thereof.
The shorter-dashed broken lines on the trapezoidal-shaped bottom portion of the wrist strap depict environmental structure and form no part of the claimed design. The longer-dashed broken lines depict stitching and form part of the claimed design.