Merge #18357: Fix missing header in sync.h

686c5456f2 Fix missing header in sync.h (João Barbosa)

Pull request description:

  `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;
  ```

ACKs for top commit:
  practicalswift:
    ACK 686c5456f2
  laanwj:
    ACK 686c5456f2

Tree-SHA512: 7c1acdfa5b0dd148d1114e14c9450d5907006e63e1a04e82ed8a1e29757925476e6f8ef6024b0c6d1bb596623115209ad580d5035be1e4785337bd01b738c9f2

src/sync.h

@@ -10,9 +10,9 @@
 #include <util/macros.h>
 
 #include <condition_variable>
-#include <thread>
 #include <mutex>
-
+#include <string>
+#include <thread>
 
 ////////////////////////////////////////////////
 //                                            //