cpu/intel/msr.h: Add license header

This is most likely an oversight. Given that the coreboot project as a
whole is licensed as GPLv2, add a GPL-2.0-only SPDX license identifier.

Change-Id: I1acaf901e1426bd6747f8a772a498a0005b457fa
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55456
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/include/cpu/intel/msr.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
 #ifndef CPU_INTEL_MSR_H
 #define CPU_INTEL_MSR_H