The ornamental design for an earplug, substantially as shown and described.
FIG. 1 is a first end perspective view of an earplug;
FIG. 2 is a first end view thereof;
FIG. 3 is a second end view thereof;
FIG. 4 is a first profile view thereof;
FIG. 5 is a second profile view thereof;
FIG. 6 is a first edge view thereof;
FIG. 7 is a second edge view thereof; and,
FIG. 8 is a second end perspective view thereof.
The broken line showing of a portion of the article is included for the purpose of illustrating subject matter excluded from the claim with the line itself and all within forming no part of the claimed design.