USB: gadget-storage needs BLOCK

With CONFIG_BLOCK=n, this build error happens:
WARNING: "bdev_read_only" [drivers/usb/gadget/g_file_storage.ko] undefined!

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>