I claim the ornamental design for a holder, as shown and described.
FIG. 1 is a front/left/top perspective view of a holder (a front/right/top perspective view would be a mirror image thereof);
FIG. 2 is a front/right/bottom perspective view thereof (a front/left/bottom perspective view would be a mirror image thereof);
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof; and,
FIG. 5 is a top plan view thereof.
The broken lines shown in the figures represent portions of the holder that form no part of the claimed design. The broken lines shown inside the holder represent environment and do not form part of the claimed design.