The ornamental design for a trinket for a kingcake, as shown and described.
FIG. 1 is a perspective view of my new design for a trinket for a king cake;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a top view thereof;
FIG. 8 is a perspective view of a trinket for a king cake, in a king cake shown in broken lines and is only to illustrate the environment and forms no part of the claimed design;
FIG. 9 is a front view thereof, showing a shelf and articles in broken lines and is only to illustrate the environment and forms no part of the claimed design; and,
FIG. 10 is a front view thereof, showing a neck and string in broken lines and is only to illustrate the environment and forms no part of the claimed design.