[C++ models] Fix compilation issues with unique_ptr under gcc 5
[C++ models] Fix compilation issues with unique_ptr under gcc 5

Summary:
`libstdc++` includes `bits/unique_ptr.h` via `include "unique_ptr.h"` in `bits/locale_conv.h` which infer can't redirect to model headers. This leads to compilation issues.
To work around that, redirect include of `locale_conv.h` to include `unique_ptr` via `include <...>` before normal `locale_conv.h`.

Reviewed By: mbouaziz

Differential Revision: D4962314

fbshipit-source-id: d4b9830

facebook/infer,blzq/infer,ropas/infer,jvillard/infer,ryoon/infer,ryoon/infer,jvillard/infer,ropas/infer,facebook/infer,ryoon/infer,blzq/infer,ryoon/infer,facebook/infer,jvillard/infer,jvillard/infer,ryoon/infer,ryoon/infer,jvillard/infer,facebook/infer,ropas/infer,facebook/infer,ropas/infer,blzq/infer,facebook/infer,ropas/infer,jvillard/infer,ropas/infer,jvillard/infer,blzq/infer,blzq/infer,facebook/infer,blzq/infer