The ornamental design for a hairbrush, as shown and described.
FIG. 1 is a bottom view of a hair brush showing my new design;
FIG. 2 is a front axial view thereof;
FIG. 3 is a rear axial view thereof;
FIG. 4 is a right axial view thereof;
FIG. 5 is a left axial view thereof; and,
FIG. 6 is a top perspective view thereof.
The broken line showing in the accompanying drawings is for the purpose of illustrating environmental features and forms no part of the claimed design.