The ornamental design for the barbeque grill accessory, as shown and described.
FIG. 1 is an isometric view of the barbeque grill accessory of this invention, showing the usage of a pair of them in combination with a barbeque grill, disclosed in hidden lines;
FIG. 2 is a front isometric view of the barbeque grill accessory of this invention;
FIG. 3 is a rear isometric view of the barbeque grill accessory, respectively;
FIG. 4 is a front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a right end view;
FIG. 7 is a left end view;
FIG. 8 is a top view; and,
FIG. 9 is a bottom view.
The broken lines depict environment and form no part of the claimed design.