The ornamental design for roof cover for a structure, as shown and described.
FIG. 1 is a front view of the roof cover for a structure.
FIG. 2 is a top view of the roof cover for a structure of FIG. 1.
FIG. 3 is a 3-3 line cross section view of the roof cover for a structure of FIG. 1; and,
FIG. 4 is a perspective view of the roof cover for a structure of FIG. 1.
A right side view, a left side view, and a rear view are not provided, as these views show identically with the front view. A bottom view is not provided as it shows identically with the plan view.