The ornamental design for a loudspeaker, as shown and described.
FIG. 1 is a perspective view of a loudspeaker showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a side elevational view of one side thereof;
FIG. 4 is a side elevational view of the opposite side thereof; and,
FIG. 5 is a top plan view thereof.
The jagged broken lines represent the bounds of the claimed design while all other broken lines are included for the purpose of illustrating portions of the loudspeaker; the broken lines form no part of the claimed design.