I claim the ornamental design for a hammer head, as shown and described.
FIG. 1 is a perspective view of a hammer head, showing my new design;
FIG. 2 is another perspective view of the hammer head of FIG. 1;
FIG. 3 is a front view of the hammer head shown in FIG. 1;
FIG. 4 is a rear view of the hammer head shown in FIG. 1;
FIG. 5 is a left side view of the hammer head shown in FIG. 1;
FIG. 6 is a right side view of the hammer head shown in FIG. 1;
FIG. 7 is a top view of the hammer head shown in FIG. 1;
FIG. 8 is a bottom view of the hammer head shown in FIG. 1; and,
FIG. 9 is another perspective view of the hammer head of FIG. 1, wherein a handle is shown in broken lines for illustrative purposes only and does not form part of the claimed design.