The ornamental design for a necklace, as shown and described.
FIG. 1 is a front right perspective view of a necklace showing my new design;
FIG. 2 is a rear left perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The necklace is shown with a symbolic break in its length. The appearance of any portion of the necklace between the break lines forms no part of the claimed design.