The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a first perspective view of a light bulb showing the claimed new design;
FIG. 2 is a second perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a top plane view thereof; and,
FIG. 5 is a bottom plane view thereof.
The broken lines immediately adjacent to the claimed portions are boundaries, all other broken lines are environmental structures; all the broken lines form no part of the claimed design.