Fix missing include

[SGauvin]

date_time.hpp was not including std_optional.hpp directly. It probably got it transitively from other files including it directly. This caused clangd to complain at every #include <toml++/toml.hpp>

What does this change do?

Adds a missing include

Is it related to an exisiting bug report or feature request?

No

Pre-merge checklist

• [x ] I've read CONTRIBUTING.md
• [x ] I've rebased my changes against the current HEAD of origin/master (if necessary)
• [ ] I've added new test cases to verify my change
• [ ] I've regenerated toml.hpp (how-to)
• [ ] I've updated any affected documentation
• [ ] I've rebuilt and run the tests with at least one of:
  • [ ] Clang 8 or higher
  • [ ] GCC 8 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [ ] I've added my name to the list of contributors in README.md