The ornamental design for a showerhead, as shown and described.
FIG. 1 is a rear perspective view of a showerhead showing my new design;
FIG. 2 is a front face view thereof;
FIG. 3 is a bottom side view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof, the opposite side being a mirror image of the side shown; and,
FIG. 6 is a top side view thereof.
The broken line showing of the rear attachment and the water holes on the face of the showerhead is included for the purpose of illustrating and forms no part of the claimed design.