The ornamental design for a handkerchief, as shown and described.
FIG. 1 is a perspective view of a handkerchief showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a perspective view thereof showing the present invention with alternate environmental structure.
The broken line showing of lettering in FIGS. 1, 2, and 7 is included for the purpose of showing environmental portions of the handkerchief and forms no part of the claimed design.