The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a top view of my new design.
FIG. 2 is a right side view. The left side view is a mirror image of the right side view.
FIG. 3 is a front end view. The rear end view is the same as the front end view; and,
FIG. 4 is a bottom view.
Dotted lines indicate unclaimed subject matter. The central section of the light bulb having diagonal shade lines is transparent, with an opaque filament extending through the central section. The pieces on either side of the central section drawn in dotted lines are opaque.