[caffe2] Avoid attempt to use undefined preprocessor directive
Summary:
This is somewhat more verbose, but it's more correct and addresses this warning on Visual Studio 2017:
```
xplat\caffe2\caffe2\core\common.h(76): warning C4067: unexpected tokens following preprocessor directive - expected a newline
```
Test Plan: Built locally with fix
Reviewed By: simpkins
Differential Revision: D28868632
fbshipit-source-id: f6a583e8275162adedb2a4bc5ed0f64847020871
