usb: gadget: f_obex: Fix sparse warning

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

drivers/usb/gadget/f_obex.c:502:21: warning: symbol 'obex_alloc' was not declared. Should it be static?
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/f_obex.c

@@ -499,7 +499,7 @@ static void obex_unbind(struct usb_configuration *c, struct usb_function *f)
 	usb_free_all_descriptors(f);
 }
 
-struct usb_function *obex_alloc(struct usb_function_instance *fi)
+static struct usb_function *obex_alloc(struct usb_function_instance *fi)
 {
 	struct f_obex	*obex;
 	struct f_serial_opts *opts;