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 top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a right side elevational view thereof, the left side being a mirror image;
FIG. 5 is a rear elevational view thereof; and,
FIG. 6 is an exploded right side elevational view thereof with a brush portion separated from a handle portion.
The broken line showing of environmental structures, namely an individual's finger, is for illustrative purposes only and forms no part of the claimed design.