staging: exfat: add missing SPDX line to Kconfig

Add SPDX identifier to Kconfig.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20190917190755.21723-1-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>