The ornamental design for a electric lightbulb, as shown and described.
FIG. 1 is a perspective view of a electric light bulb showing our new design, with the broken lines showing for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view thereof;
FIG. 4 is a rear elevational view thereof; and,
FIG. 5 is a opposite side elevational view thereof.