We claim the ornamental design for a banner holder, as shown and described.
FIG. 1 is a front perspective view thereof;
FIG. 2 is an exploded front perspective view of FIG. 1;
FIG. 3 is a left-side elevational view of FIG. 1;
FIG. 4 is a right-side elevational view of FIG. 1;
FIG. 5 is a bottom plan view of FIG. 1, the top plan view being a mirror image thereof;
FIG. 6 is a front elevational view of FIG. 1; and,
FIG. 7 is a rear elevational view of FIG. 1.
The broken lines showing a banner depict environmental matter and form no part of the claimed design. The broken lines in a central portion of the banner holder depict portions of the banner holder which form no part of the claimed design.