MdePkg: Fix Clang build failure

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gao, Liming <liming.gao@intel.com>
Reviewed-by: Andrew Fish <afish@apple.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15784 6f19259b-4bc3-4df7-8a09-765794883524