The ornamental design for a wristwatch, as shown and described.
1. Wristwatch
Representation 1.1 is a front view of the wristwatch; representation 1.2 is a rear view of the wristwatch; representation 1.3 is a top view of the wristwatch; representation 1.4 is a bottom view of the wristwatch; representation 1.5 is a left-side view of the wristwatch; representation 1.6 is a right-side view of the wristwatch; representation 1.7 is a perspective view of the wristwatch.