MdePkg: Fix ACPI NFIT GUID definitions
    
    Fix ACPI 6.0 and 6.1 NFIT GUID definitions to avoid GCC compiler error
    for missing braces.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Samer El-Haj-Mahmoud <elhaj@hpe.com>
    Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>