The ornamental design for a switch for a scanner, as shown and described.
FIG. 1 is a front perspective view of a switch for a scanner showing our new design in a usage configuration wherein the transparent lid is in a forward position;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a front perspective view thereof in a usage configuration wherein the transparent lid is in a rear position.