The ornamental design for a grillbrush, as shown and described.
FIG. 1 is a perspective view of a grill brush of the present invention;
FIG. 2 is a top view thereof;
FIG. 3 is a right side view thereof, the left side being a mirror image; and,
FIG. 4 is a bottom view thereof.
The broken lines shown on the head and the handle in FIGS. 1 and 3 illustrate portions of the grill brush that form no part of the claimed design. The shaft portion is shown broken away in FIGS. 2-4 for ease of illustration.