The ornamental design for a playing card holder, as shown and described.
FIG. 1 is a perspective view of a playing card holder showing my new design, the broken lines showing playing cards are for illustrative purposes only and form no part of the claimed design;
FIG. 2 is a front view thereof; and,
FIG. 3 is a left side elevational view thereof, the right side being a mirror image.
The rear of the playing card holder is flat and unornamented.