MINOR: [C++][CI] Fix s3fs_test.cc compile error
MINOR: [C++][CI] Fix s3fs_test.cc compile error

Fix this error:
```
C:\Miniconda37-x64\envs\arrow\Library\include\boost/process/detail/traits/wchar_t.hpp(150): error C2039: 'transform': is not a member of 'std'
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\locale(20): note: see declaration of 'std'
C:\Miniconda37-x64\envs\arrow\Library\include\boost/process/detail/traits/wchar_t.hpp(150): error C3861: 'transform': identifier not found
```

.. seen on AppVeyor:
https://ci.appveyor.com/project/ApacheSoftwareFoundation/arrow/builds/40350532/job/xr1qhmak9jtdn7rl#L1217

The underlying problem may be a missing include in boost/process.

Closes #10925 from pitrou/fix-appveyor-s3fs

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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