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 top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a right side view thereof.
The broken line showing of the water spray openings in FIGS. 1 and 3, is included for the purpose of illustrating and forms no part of the claimed design.