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 thereof;
FIG. 3 is a right side view thereof, with the left, top and bottom views being a mirror image;
FIG. 4 is a front elevational view thereof, shown on a hand-held shower; and,
FIG. 5 is a right side view thereof, with the left, top and bottom views being a mirror image.
The broken line showing of a wall mount showerhead body in FIGS. 1 and 3 and a hand-held body in FIGS. 4 and 5 is included for the purpose of illustrating and forms no part of the claimed design.