The ornamental design for a hairbrush, as shown and described.
FIG. 1 is a bottom front perspective view of a new design for a hair brush according to the present invention;
FIG. 2 is a top front perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a first side elevational view thereof, wherein a second side elevational view is identical to and opposite of the first side elevational view;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in the drawings illustrate portions of the hair brush that form no part of the claimed design