arch/x86: x86_64: Fix generated include

Commit d78bf6c16580 ("kconfig: Deprecate LEGACY_GENERATED_INCLUDE_PATH")
made it mandatory that generated includes need to include the
"zephyr" namespace, which was missed for a shadow stack related
include on `locore.S`.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>