The ornamental design for a signholder, as shown and described.
FIG. 1 is a front, top, and left perspective view of a sign holder in accordance with the present disclosure;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a back elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a front, bottom, and left perspective view thereof.
In the drawings, the broken lines depict unclaimed subject matter only and form no part of the claimed design. The separation and brackets indicate that the precise length of the sign holder is not claimed.