The ornamental design for a swimsuit, as shown and described.
FIG. 1 is a front right perspective view of a swimsuit;
FIG. 2 is a front left perspective view thereof;
FIG. 3 is a rear left perspective view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a front left perspective view thereof.
The broken line shown in the relevant drawings shows unclaimed subject matter for the design in those drawings. The blue colored body outline also reflects environmental matter not part of the claim.