Fix multiple definitions of Listener2D

(cherry picked from commit 6b68445e4d83684fe79ebfb5d5cbfc03a87153c7)

scene/main/viewport.cpp

@@ -35,7 +35,7 @@
 #include "core/os/os.h"
 #include "core/project_settings.h"
 #include "scene/2d/collision_object_2d.h"
-#include "scene/2d/listener_2d.cpp"
+#include "scene/2d/listener_2d.h"
 #include "scene/3d/camera.h"
 #include "scene/3d/collision_object.h"
 #include "scene/3d/listener.h"