The ornamental design for a loudspeaker, as shown and described.
1.-2. Loudspeaker
1.1 : Front
1.2 : Back
1.3 : Left
1.4 : Right
1.5 :Top
1.6 : Bottom
1.7 : Reference view
2.1 : Front
2.2 : Back
2.3 : Left
2.4 : Right
2.5 : Top
2.6 : Bottom
The broken lines shown in the drawings illustrate portions of the loudspeaker that form no part of the claimed design; the reproduction 1.7 is a reference view and forms no part of the claimed design.