[yaml2elf.cpp] - Fix compilation under linux.

Fixes errors like:
/home/ssglocal/clang-cmake-x86_64-sde-avx512-linux/clang-cmake-x86_64-sde-avx512-linux/llvm/tools/yaml2obj/yaml2elf.cpp:597:5: error: need ‘typename’ before ‘ELFT:: Xword’ because ‘ELFT’ is a dependent scope
     ELFT::Xword Tag = (ELFT::Xword)DE.Tag;

llvm-svn: 353614