The ornamental design for a hairbrush, as shown and described.
FIG. 1 is a perspective view of a hair brush showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The claim is understood to end with the solid line defining the base of the uppermost cylindrical portion of the handle. The broken lines showing the remainder of the handle and part of the top of the head illustrate portions of hair brush that form no part of the claimed design.