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

drivers/staging/exfat/Kconfig

+# SPDX-License-Identifier: GPL-2.0
 config EXFAT_FS
 	tristate "exFAT fs support"
 	depends on BLOCK