mei: add missing SPDX tag to mei Kconfig

Add SPDX tag with GPLv2 license to mei Kconfig.
Signed-off-by: NTomas Winkler <tomas.winkler@intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/Kconfig

+# SPDX-License-Identifier: GPL-2.0
 config INTEL_MEI
 	tristate "Intel Management Engine Interface"
 	depends on X86 && PCI