The ornamental design for “signboard”, as shown and described.
FIG. 1 is a perspective view of a signboard showing my new design;
FIG. 2 is a front elevational view of the signboard illustrated by FIG. 1;
FIG. 3 is a side perspective view of the signboard illustrated by FIG. 1, showing the data carrier element oriented at an angle relative to the supporting base;
FIG. 4 is a front elevational view of a signboard illustrated by FIG. 1 in which the data carrier element is substantially perpendicular to the supporting base in an alternate position of use; and,
FIG. 5 is a side perspective view of the signboard illustrated by FIG. 4.