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

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[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