Add the ohos macro to the cmakelist,cpp and h files for OpenHarmony cross compilation

## What, How & Why? <!-- Describe the changes and give some hints to guide your reviewers if possible. --> <!-- Link to relevant issue this fixes --> we are using realm-core lib in OpenHarmony, so we'd like to contribute the build guide, hope it can help developers. ## ☑️ ToDos * [ ] 📝 Changelog update * [ ] 🚦 Tests (or not relevant) * [ ] C-API, if public C++ API changed * [ ] `bindgen/spec.yml`, if public C++ API changed