cpu/intel/fsp_model_406dx: Invaild include path

Found-by: Clang
Change-Id: Iac54755caadc3ffbe8a09d40aed8500c2359e829
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7349
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>