The ornamental design for a signholder, as shown and described.
FIG. 1 is a perspective view of the sign holder from the front, above and to the right;
FIG. 2 is a perspective view of the sign holder from the front, above and to the left;
FIG. 3 is a view of the sign holder from the front; the rear view being a mirror image thereof;
FIG. 4 is a right side view, the left-side being a mirror image thereof;
FIG. 5 is a top view; and,
FIG. 6 is a bottom view.
The broken line showing of the sign is for the purpose of illustrating environment and forms no part of the claimed design. The broken line showing of elements on the sign holder illustrate portions of the article and form no part of the claimed design.