The ornamental design for a headphone, as shown and described.
1. Headphone
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Right
1.5 : Left
1.6 : Top
1.7 : Bottom
1.8 : Cross-sectional
1.9 : Perspective
FIG. 1.8 shows a cross-section of an upper part of an arm, which extends from headband to ear-cup of the headphones; the section is taken about X-X shown in FIG. 1.4; the parallel fine lines shown in the drawings indicate the relief of the surface of the article, not surface decoration and the broken lines indicate portion(s) of the article that form no part of the claimed design; and FIG. 1.9 provides an enlarged view of the part of the headphone circled by the broken line in FIG. 1.1.