The ornamental design for an earring, as shown and described.
FIG. 1 is a front view of an earring showing my new design;
FIG. 2 is a back view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a top view thereof.
The seventeen small circles and twenty arrow-like features in FIG. 1 are substantially flat with no depth, contour or indentation. The broken lines depict portions of the earring that form no part of the claimed design.