The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a front perspective view showing a light bulb of the present invention;
FIG. 2 is a front view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a bottom plan view thereof.
In the drawings, the broken line showings are for the purpose of illustrating portions of a light bulb or environmental structure and forms no part of the claimed design.