USB: audio: guard kernel-only code with __KERNEL__

include/linux/usb/audio.h is exported to userspace,
so part of this file that is for internal kernel
usage need to be guarded with ifdef __KERNEL__.
This way make headers_install will stript it out.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/usb/audio.h

@@ -276,6 +276,8 @@ struct uac_iso_endpoint_descriptor {
 #define UAC_FU_BASS_BOOST	(1 << (UAC_BASS_BOOST_CONTROL - 1))
 #define UAC_FU_LOUDNESS		(1 << (UAC_LOUDNESS_CONTROL - 1))
 
+#ifdef __KERNEL__
+
 struct usb_audio_control {
 	struct list_head list;
 	const char *name;
@@ -294,4 +296,6 @@ struct usb_audio_control_selector {
 	struct usb_descriptor_header *desc;
 };
 
+#endif /* __KERNEL__ */
+
 #endif /* __LINUX_USB_AUDIO_H */