The ornamental design for a washbasin, as shown and described.
FIG. 1 is a top perspective view of a washbasin showing my new design;
FIG. 2 is a top view of the washbasin;
FIG. 3 is a left side view of the washbasin;
FIG. 4 is a bottom view of the washbasin;
FIG. 5 is a bottom perspective view of the washbasin; and,
FIG. 6 is a cross-sectional view of the washbasin along line A-A.
Right side, front and rear views not shown are mirror images of the left side view. Hatch lines in FIG. 6 depict internal structure not visible from the exterior that do not form part of the design sought to be patented.