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 elevation view thereof;
FIG. 4 is a right side elevation 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 dashed broken lines shown in the drawings depict environmental structure forming no part of the claimed design. The long-short-long broken lines illustrate the bounds of the enlarged portions of FIGS. 5-6 as well as the bounds of the enlarged figures, FIGS. 8-9 and form no part of the claimed design.