Modify CYaml header includes for Swift C++ compatibility

[johnfairh]

Moving a large Swift-C++ project to Xcode 26 / macOS 15 / arm I hit this error in Xcode about CYaml/yaml.h:

I found this forum post from one of the Apple Swift-C++ developers, made the suggested change, and my projects built again.

[BruceMcRooster]

I had the same issue, so +1 to merging this! By messing around it appears that just moving #include <string.h> outside of extern "C" is the minimal requirement to make this work. Of course, yours just looks cleaner. I believe this is related to this restriction, but I'm not sure why string.h would fall under this (it's not a Clang module).