The ornamental design for a sideboard, as shown and described.
FIG. 1 is a left front perspective view of a sideboard showing my new design; the top having been shown broken line boundary to indicate that the surface ornamentation of the top forms no part of the claimed design. A similarly broken line boundary on the other figures has the same significance.
FIG. 2 is a side perspective view, the opposite side being a mirror image thereof;