The ornamental design for an earplug, as shown and described.
FIG. 1 is a perspective view of an earplug in accordance with the claimed design;
FIG. 2 is a front elevation view of the design of FIG. 1, the rear elevation view being a mirror image thereof;
FIG. 3 is a top plan view of the design of FIG. 1; and,
FIG. 4 is a bottom plan view of the design of FIG. 1.
The dash-dot break lines in the figures are used to indicate indeterminate length of the element in which the break lines are provided. The break lines form no part of the claimed design. The dashed broken lines are used to show elements of the article excluded from the claim.