The ornamental design for a signheader, as shown and described.
FIG. 1 is an isometric view of a sign header;
FIG. 2 is a right end view thereof;
FIG. 3 is a left end view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a back view thereof; and,
FIG. 7 is a front view thereof.
In the drawings, the evenly spaced broken lines represent environment and features, which form no part of the claimed design. The dash-dot dash broken lines define the boundaries of the claim and form no part thereof.