Fix build msys2

[Toni500github]

What does this change do?
it fixes a compilation error from windows msys2, because looks like impl::widen(file_path_str).c_str() returns a const wchar_t* which seems to cause the errors.
I had a workaround to put that TOML_WINDOWS is false and it compiled, so I've decided to remove it.

Is it related to an exisiting bug report or feature request?
no

Pre-merge checklist

• I've read CONTRIBUTING.md
• 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

[marzer]

Much better. Thanks 😄 🎉