The ornamental design for a wristwatch, as shown and described.
FIG. 1 is front elevational view of a wristwatch, showing my new design.
FIG. 2 is a right side elevational view thereof.
FIG. 3 is a front right perspective view thereof; and,
FIG. 4 is a rear right perspective view thereof.
The faded area of the wristwatch shown in FIG. 4 and outlined in dot-dash lines illustrates portions of the wristwatch that form no part of the claimed design.