The ornamental design for a brushholder, as shown and described.
FIG. 1 is a front elevational view of a brush holder showing my new design, the back elevational view being a mirror-image thereof;
FIG. 2 is a right side elevational view thereof, the left side view being a mirror-image thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof; and,
FIG. 5 is a perspective view thereof.
The broken lines showing an artist's brush, a pencil and a portion of the brush holder in FIGS. 1, 2, and 4 are included for the purposes of showing articles in the environment and a portion of the brush holder that form no part of the claimed design.