The ornamental design for a hammerhead, as shown and described.
FIG. 1 is a front top perspective view of a hammerhead, showing my new design;
FIG. 2 is another front top perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a front perspective view thereof.
The broken lines showing portions of the hammerhead and a hammer handle are included for environmental purposes only and form no part of the claimed design.