Fix gcc-specific problem in reentrant_allocator test
Fix gcc-specific problem in reentrant_allocator test

Summary:
[Folly] Fix gcc-specific problem in `reentrant_allocator` test.

```
folly/memory/test/ReentrantAllocatorTest.cpp: In member function ‘virtual void ReentrantAllocatorTest_large_Test::TestBody()’:
folly/folly/memory/test/ReentrantAllocatorTest.cpp:68:22: error: no matching function for call to ‘std::vector<ReentrantAllocatorTest_large_Test::TestBody()::type, folly::reentrant_allocator<ReentrantAllocatorTest_large_Test::TestBody()::type> >::push_back(<brace-enclosed initializer list>)’
     vec.push_back(type{i});
                      ^
```

Differential Revision: D19738608

fbshipit-source-id: bbffd1f47749be1aa88bb45bddc401ffca48c4ca

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly