usb: gadget: dummy: fix sparse warning

fix the following sparse warning:

drivers/usb/gadget/dummy_hcd.c:1912:12: warning: symbol 'usb3_bos_desc' was not declared. Should it be static?
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/dummy_hcd.c

@@ -1906,7 +1906,7 @@ static int dummy_hub_status(struct usb_hcd *hcd, char *buf)
 }
 
 /* usb 3.0 root hub device descriptor */
-struct {
+static struct {
 	struct usb_bos_descriptor bos;
 	struct usb_ss_cap_descriptor ss_cap;
 } __packed usb3_bos_desc = {