The ornamental design for a hairextension, as shown and described.
FIG. 1 is a front view of a hair extension in a stretched out configuration showing my new design;
FIG. 2 is a back view of the hair extension in the stretched out configuration thereof; and,
FIG. 3 is a perspective view of the hair extension secured with a bundle wrap.
The broken lines in FIG. 3 represent a bundle wrap to secure the hair extension in a wrapped configuration for packaging and depict the boundary of the claim, and it is understood that the claim extends to the boundary, but the boundary forms no part of the claim.