The ornamental design for a hairbrush, as shown and described.
FIG. 1 is a front elevational view of a hairbrush showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view thereof.
FIG. 8 is an enlarged view of FIG. 5; and,
FIG. 9 is an enlarged view of FIG. 6.
The broken lines drawn with uniform, short line segments illustrates environmental structure. The broken lines drawn with short and long line segments illustrate the bounds of the enlarged detail views. None of the drawing elements shown in broken lines form a part of the claimed design.