C++ SDK: add missing include for <string> (#7321)

### What

When building 0.18.2 on windows with clang-cl it fails because of
undefined `std::string`. With gcc in ubuntu 24.04 it builds fine even
without the include, but I still consider this is the right thing to do.
I have not tested other compilers.

The setup is simply:
```cmake
include(FetchContent)
FetchContent_Declare(rerun_sdk
    URL https://github.com/rerun-io/rerun/releases/download/0.18.2/rerun_cpp_sdk.zip
)
FetchContent_MakeAvailable(rerun_sdk)

```

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7321?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/7321?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!
* [x] If have noted any breaking changes to the log API in
`CHANGELOG.md` and the migration guide

- [PR Build Summary](https://build.rerun.io/pr/7321)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

To run all checks from `main`, comment on the PR with `@rerun-bot
full-check`.

---------

Co-authored-by: Jeremy Leibs <[email protected]>

rerun_cpp/src/rerun/entity_path.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <string>
 #include <string_view>
 #include <vector>