The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a perspective view of a lighting fixture showing my design;
FIG. 2 is a front elevational thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dash-dot-dash broken lines shown represent the bounds of the claim and forms no part thereof and the dashed broken lines shown therein illustrate portions of the lamp that form no part of the claimed design.