MdePkg Smbios.h: Update spec version info to 3.1.1
    
    It was missed to be updated at
    SHA-1: 043026ac12c29703bb7e3c19b9ae5e8177bb554b.
    
    Cc: Liming Gao <liming.gao@intel.com>
    Cc: Michael Kinney <michael.d.kinney@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Star Zeng <star.zeng@intel.com>
    Reviewed-by: Liming Gao <liming.gao@intel.com>