usb: gadget: f_ncm: Fix sparse warning

Make local symbol static in order to fix the following sparse
warning.

drivers/usb/gadget/f_ncm.c:1389:21: warning: symbol 'ncm_alloc' was not declared. Should it be static?
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_ncm.c

@@ -1386,7 +1386,7 @@ static void ncm_unbind(struct usb_configuration *c, struct usb_function *f)
 	usb_ep_free_request(ncm->notify, ncm->notify_req);
 }
 
-struct usb_function *ncm_alloc(struct usb_function_instance *fi)
+static struct usb_function *ncm_alloc(struct usb_function_instance *fi)
 {
 	struct f_ncm		*ncm;
 	struct f_ncm_opts	*opts;