The ornamental design for a stirrup, as shown and described.
FIG. 1 is a front perspective view of a stirrup showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of the foot surface and indented side pieces below the foot surface of the stirrup show portions of a stirrup and form no part of the claimed design.