I claim the ornamental design for a cosmetic brush, as shown and described.
FIG. 1 is a perspective view of cosmetic brush;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 a bottom plan view thereof.
The broken line showing a brush head and a ferrule is included for the purpose of illustrating use and environment and forms no part of the claimed design.