fix build with mingw g++ 10

[claudeha]

What does this change do?

Adds c_str() for compatibility with mingw, which doesn't have wstring overload for open(), but does have a wchar_t * overload.

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

Fixes: https://github.com/marzer/tomlplusplus/issues/182

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)
  • [X] (I made duplicate changes by hand...)
• [ ] I've updated any affected documentation
• [ ] I've rebuilt and run the tests with at least one of:
  • [ ] Clang 6 or higher
  • [ ] GCC 7 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [ ] I've added my name to the list of contributors in README.md

[marzer]

Thanks!