caffe2_benchmark msvc build fix (#15619)
Summary:
Fixing error in caffe2_benchmark binary
```
2018-12-29T14:09:59.7867995Z d:\a\1\s\caffe2_builders\v141\pytorch\binaries\benchmark_helper.h(90): error C2678: binary '|=': no operator found which takes a left-hand operand of type 'std::_Iosb<int>::_Openmode' (or there is no acceptable conversion) (compiling source file D:\a\1\s\caffe2_builders\v141\pytorch\binaries\benchmark_helper.cc) [D:\a\1\s\caffe2_builders\v141\pytorch\build\Release\binaries\caffe2_benchmark.vcxproj]
2018-12-29T14:09:59.7868252Z d:\a\1\s\caffe2_builders\v141\pytorch\binaries\benchmark_helper.h(92): error C2678: binary '|=': no operator found which takes a left-hand operand of type 'std::_Iosb<int>::_Openmode' (or there is no acceptable conversion) (compiling source file D:\a\1\s\caffe2_builders\v141\pytorch\binaries\benchmark_helper.cc) [D:\a\1\s\caffe2_builders\v141\pytorch\build\Release\binaries\caffe2_benchmark.vcxproj]
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15619
Differential Revision: D13580195
Pulled By: soumith
fbshipit-source-id: b0a4479cd5f7555801b1977aeee96b6433293da7
