We claim the ornamental design for a megaphone, as shown and described.
FIG. 1 is a front and right side perspective view of a megaphone, embodying our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dot-dot-dash broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are included for the purpose of illustrating portions of the megaphone; the broken lines form no part of the claimed design.