[C++] Missing sstream include in cpp/src/arrow/util/string_util.h

[atupone]

Describe the bug, including details regarding any error messages, version, and platform.

std::unique_ptrstd::ostringstream sstream_; require sstream include.

The bug has been raised while building with gcc-17.

https://bugs.gentoo.org/976604

Component(s)

C++