The ornamental design for a joystick, as shown and described.
FIG. 1 is a front view of a joystick showing my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof; and,
FIG. 7 is a perspective view thereof.
The dash-dotted broken lines in the drawings define the bounds of the claimed design; the evenly spaced broken lines show portions of the joystick; all broken lines form no part of the claimed design.