Fix C++ string include
This file uses functions like `strrchr` that are provided by string.h,
it seems like somehow this works implicitly on macOS but this seems to
be more correct.
Closes
#include <libgen.h>
#include <spawn.h>
+#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
+
#include <cerrno>
#include <climits>
#include <cstdio>
#include <map>
#include <memory>
#include <sstream>
-#include <string>
#include <utility>
#include <vector>
