The ornamental design for a headphone, as shown and described.
FIG. 1 is a perspective view of a headphone, showing the new design;
FIG. 2 is a front elevational view of the headphone;
FIG. 3 is a rear elevational view of the headphone;
FIG. 4 is a top plan view of the headphone;
FIG. 5 is a bottom plan view of the headphone;
FIG. 6 is a left side elevational view of the headphone;
FIG. 7 is a right side elevational view of the headphone;
FIG. 8 is a sectional view of the headphone taken in the direction of line 8-8 of FIG. 2; and,
FIG. 9 is an enlarged partial sectional view of the headphone taken in the direction of line 9-9 of FIG. 4.
The broken lines shown in the drawings depict portions of the headphone that form no part of the claimed design. The dot-dash line depicts a boundary of the claim and forms no part thereof.