subprocess: Fix -Wunused-private-field for Popen class on Windows

When compiling with clang-cl on Windows, `src/util/subprocess.h` emits
`-Wunused-private-field` warnings about unused private fields in the
`Popen` class.

src/util/subprocess.h

@@ -1024,6 +1024,9 @@ private:
 #ifdef __USING_WINDOWS__
   HANDLE process_handle_;
   std::future<void> cleanup_future_;
+#else
+  // Pid of the child process
+  int child_pid_ = -1;
 #endif
 
   std::string exe_name_;
@@ -1034,9 +1037,6 @@ private:
   std::vector<std::string> vargs_;
   std::vector<char*> cargv_;
 
-  // Pid of the child process
-  int child_pid_ = -1;
-
   int retcode_ = -1;
 };