The ornamental design for a platform, as shown and described.
FIG. 1 is a perspective view of a platform showing our new design;
FIG. 2 is a left side view thereof, the right side view being a mirror image of the left side view;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a side view of the platform in its environment.
The dashed line showing of structural lines is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.