The ornamental design for a windshieldbanner, as shown and described.
FIG. 1 is an environmental perspective view of a windshield banner, showing my new design;
FIG. 2 is a front view thereof, the back view being a mirror image;
FIG. 3 is a top plan view thereof;
FIG. 4 is a side elevational view thereof, the other side being a mirror image; and,
FIG. 5 is a bottom plan view thereof.
The broken line showings of a car in FIG. 1 and the supportive structure in all views is for illustrative purposes only and forms no part of the claimed design.