The ornamental design for a loudspeaker, as shown and described.
FIG. 1 is a perspective view of a loudspeaker;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view thereof. The broken line showing of another loudspeaker is for illustrative purposes only and forms no part of the claimed design.