MINOR: [C++] Add missing header <type_traits>

[wgtmac]

What is the purpose of the change

Fix avro-cpp builds with C++23 by including <type_traits> directly in Buffer.hh, which uses std::is_fundamental.

Verifying this change

This change is already covered by existing tests, such as the avro-cpp C++23 build and existing C++ test suite.

• Built avrocpp_s with -DCMAKE_CXX_STANDARD=23
• Ran the avro-cpp C++ test suite: 13/13 tests passed

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[wgtmac]

I tried to bump Avro-cpp in iceberg-cpp (which uses C++23) but failed due to a missing header. Could you help take a look? @martin-g @Fokko