USB: move <linux/usb_gadget.h> to <linux/usb/gadget.h>

Move <linux/usb_gadget.h> to <linux/usb/gadget.h>, reducing
some of the clutter in the main include directory.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/gadget/serial.c b/drivers/usb/gadget/serial.c
index a2a1ebc..f5738eb 100644
--- a/drivers/usb/gadget/serial.c
+++ b/drivers/usb/gadget/serial.c
@@ -25,7 +25,7 @@
 
 #include <linux/usb/ch9.h>
 #include <linux/usb/cdc.h>
-#include <linux/usb_gadget.h>
+#include <linux/usb/gadget.h>
 
 #include "gadget_chips.h"