The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a upper- front prospective view of the light bulb.
FIG. 2 is a front or rear view of the light bulb of FIG. 1, with the broken line showing for illustrative purposes only and forming no part of the claimed design;
FIG. 3 is a top view of the light bulb of FIG. 1.
FIG. 4 is a bottom view of the light bulb of FIG. 1; and,
FIG. 5 is a side view of the light bulb of FIG. 1.