Fixed MSVC build with CL.exe 19.41.34120

[aheysse]

<string> is no longer automatically included in the various header files thread.h uses when compiling with MSVC/cl.exe. I've explicitly included it now so MSVC/cl.exe builds work again.

[DevFelixFaber]

Suffering from the same issue. We keep our tools up-to-date, but now we can't easily include this library using Conan as the compilation fails due to this error. Would love to see this fix merged into the next release :)

[David-Haim]

fixed with #175