Add IAP_BUFFERED preprocessor macro to filter out new work to put iAP through buffer and decouple.

module_usb_aud_shared/usb_buffer/decouple.h

@@ -13,6 +13,10 @@
  */
 void decouple(chanend c_audio_out,
               chanend ?c_led,
-              chanend ?c_midi, chanend ?c_clk_int, chanend ?c_iap);
+              chanend ?c_midi, chanend ?c_clk_int
+#ifdef IAP_BUFFERED
+, chanend ?c_iap
+#endif
+);
 
 #endif // __decouple_h__