The ornamental design for a lighting device, as shown and described.
FIG. 1 is a front perspective view showing my new design of a lighting device. The housing of the lighting device is symmetrical front to back, and left to right.
FIG. 2 is a right-hand perspective view of the design shown in FIG. 1; and,
FIG. 3 is a further perspective view of the design shown in FIG. 1.