The ornamental design for a wristwatch, as shown and described.
FIG. 1 is a top, front and side perspective view of a wristwatch showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view.
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 invention.