usb: gadget: FunctionFS: convert to new function interface with backward compatibility

This is required in order to integrate configfs support.
f_fs needs to be a separately compiled module and so it needs to use the new
interface.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/include/linux/usb/functionfs.h b/include/linux/usb/functionfs.h
index 9c1e926..3448efb 100644
--- a/include/linux/usb/functionfs.h
+++ b/include/linux/usb/functionfs.h
@@ -3,6 +3,7 @@
 
 #include <uapi/linux/usb/functionfs.h>
 
+#ifdef USB_FFS_INCLUDED
 
 struct ffs_data;
 struct usb_composite_dev;
@@ -20,3 +21,4 @@
 
 
 #endif
+#endif