StandaloneMmPkg: Fix ECC error 4002 in StandaloneMmCpu
    
    Bugzilla: 3150 (https://bugzilla.tianocore.org/show_bug.cgi?id=3150)
    
    Fix the ECC reported error "[4002] Function header doesn't exist".
    
    Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
    Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
    Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>