Fix missing header in sync.h #18357

std::string is referenced in sync.h but the relevant header is not explicitly included as required by current guideline. Furthermore on osx 10.14.6 with clang-900.0.31 the following error occurs:

In file included from threadinterrupt.cpp:6:
In file included from ./threadinterrupt.h:8:
./sync.h:206:21: error: implicit instantiation of undefined template 'std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
        std::string lockname;

ACK 686c5456f2fcf7e301907223d16a85f7eb378c6c

@promag

My macOS 10.14 reports:

$ clang --version
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.0.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

... on osx 10.14.6 with clang-900.0.31...

Is your clang from Apple or from brew?

@hebasto

$  clang -v
Apple LLVM version 9.0.0 (clang-900.0.31)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

ACK 686c5456f2fcf7e301907223d16a85f7eb378c6c