The ornamental design for a wristwatch, as shown and described.
FIG. 1 is a top, front and side perspective view of a wrist watch showing my new design;
FIG. 2 is a bottom, rear and side perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawing figures are used to show portions of the wristwatch that are not being claimed, and thus form no part of the claimed design.