The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a perspective view showing a light bulb.
FIG. 2 is a front view of a light bulb.
FIG. 3 is a rear view of a light bulb.
FIG. 4 is a top view of a light bulb.
FIG. 5 is a bottom view of a light bulb.
FIG. 6 is a side view of a light bulb; and,
FIG. 7 is a side view of a light bulb.
The broken line showing of the bottom portion of the light bulb is included for the purpose of illustrating only and forms no part of the claimed design.