[lib/ObjectYAML] - Fix BB after r353607 [2]. NFC.

The second and the last place it seems.

Error was:
[  4%] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Error.cpp.o
/Users/buildslave/as-bldslv9_new/lld-x86_64-darwin13/llvm.src/lib/ObjectYAML/ELFYAML.cpp:993:15: error: unused variable 'Object' [-Werror,-Wunused-variable]
  const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());

llvm-svn: 353609