fix(example): c++14 limitation (#19292)

fix the bug:

```shell
‘std::string_view’ is only available from C++17 onwards
```

The error occurs because the code is being compiled using the C++14 standard, which does not include the std::string_view type introduced in C++17. The Abseil library (absl), used by Protocol Buffers, includes absl/strings/string_view.h, which uses std::string_view. This causes the compilation to fail in environments configured with C++14 or earlier.

In the provided build system, the file add_person.cc is being compiled using the -std=c++14 flag, which is incompatible with code depending on features from C++17 (e.g., std::string_view).

Closes #19292

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/19292 from Kaikaikaifang:patch-1 ee276ffd0b130024a408526366c63d46889e2118
PiperOrigin-RevId: 698223950