The ornamental design for a hairbrush, substantially as shown and described.
FIG. 1 is a perspective view of the hair brush.
FIG. 2 is bottom view of the hair brush.
FIG. 3 is top view of the hair brush.
FIG. 4 is right side view of the hair brush.
FIG. 5 is left side view of the hair brush.
FIG. 6 is front view of the hair brush; and,
FIG. 7 is back view of the hair brush.
The broken lines showing the bristles as well as the transition between the handle and the brush head illustrate portions of the hair brush that form no part of the claimed design.