fix: add missing cstdlib header in cli_args.cpp

[karan-singare]

What, How & Why?

Fix compilation issue in realm-core by adding missing cstdlib header. This resolves a build compatibility problem where std::strtol function was used without including the proper header declaration, causing compilation failures in some environments.

The fix ensures the code compiles correctly across different platforms and compiler versions by providing the proper declaration for the std::strtol function used on line 67 of cli_args.cpp.

This closes #7021

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 📝 Update COMPATIBILITY.md
• 🚦 Tests
• 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• 📱 Check the React Native/other sample apps work if necessary
• 💥 Breaking label has been applied or is not necessary
• 🔔 Mention @realm/devdocs if documentation changes are needed

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Karan Singh Singare.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails

[kraenhansen]

This reference a commit in Realm Core which doesn't exist? realm/realm-core@b8b65b0

I do see you've created this PR in Realm Core: realm/realm-core#8089