The ornamental design for a showerhead, as shown and described.
FIG. 1 is a perspective view of a showerhead showing my new design;
FIG. 2 is a front elevational view of the showerhead showing my new design; and,
FIG. 3 is a right side elevational view of the showerhead showing a bell-shaped body showing my new design, with a left side elevational view, a top view and a bottom view being mirror images thereof.
The broken line showing is included for the purpose of illustrating and forms no part of the claimed design.