The ornamental design for a holder, as shown and described.
FIG. 1 is a perspective view of a holder showing my new design attached to the back of a light bar of a clipboard environment;
FIG. 2 is a top plan view showing my holder design, the bottom plan view being a mirror image;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a side elevation view of one side thereof; and,
FIG. 5 is a side elevation view of another side thereof.
The broken lines showing a clipboard, pen, and the inside of the holder are included for the purpose of illustrating environment and form no part of the claimed design.