I claim the ornamental design for a refill reservoir, as shown and described.
FIG. 1 is a top front perspective view of a refill reservoir according to my new design;
FIG. 2 is a front view of the refill reservoir shown in FIG. 1;
FIG. 3 is a rear view of the refill reservoir shown in FIG. 1;
FIG. 4 is a right side view of the refill reservoir shown in FIG. 1;
FIG. 5 is a left side view of the refill reservoir shown in FIG. 1;
FIG. 6 is a top view of the refill reservoir shown in FIG. 1; and,
FIG. 7 is a bottom view of the refill reservoir shown in FIG. 1.
The ornamental design which is claimed is shown in solid lines in the drawings: The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.