The ornamental design for a horn for loudspeaker, as shown and described.
FIG. 1 is a right rear side perspective view of a horn for loudspeaker showing my new design;
FIG. 2 is a front left side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a top view thereof.
Those portions of the figures that are presented in broken lines merely illustrate the environment of the design and are not part of the claimed design.