The ornamental design for a lightsource, as shown and described.
FIG. 1 is a perspective view of a light source according to the present invention.
FIG. 2 is a top plan view of the light source.
FIG. 3 is a front elevational view of the light source, with the rear elevational view being identical to the front elevational view.
FIG. 4 is a left side elevational view of the light source, with the right side elevational view being identical to the left side elevational view; and,
FIG. 5 is a bottom plan view of the light source.
The broken lines shown in the drawings are for the purpose of illustrating unclaimed portions of the light source and form no part of the claimed design.