The ornamental design for a platform, as shown and described.
FIG. 1 is a top view of the platform of the present invention;
FIG. 2 is a longitudinal side view of the platform of the present invention;
FIG. 3 is a lateral side view of the platform of the present invention; and,
FIG. 4 is a perspective view of the platform of the present invention.
The broken lines in the figures, showing all components of the platform except for the longitudinal side component shown in FIG. 2 and FIG. 4 in solid lines, are for illustrative purposes only and form no part of the claimed design.