Replace sstream header with iosfwd in headers (#2712)

The latter is smaller and sufficient for parameters.

src/cache_node.cpp

@@ -19,6 +19,7 @@
  */
 
 #include <iomanip>
+#include <sstream>
 
 #include "s3fs.h"
 #include "s3fs_logger.h"

src/cache_node.h

@@ -21,8 +21,8 @@
 #ifndef S3FS_CACHE_NODE_H_
 #define S3FS_CACHE_NODE_H_
 
+#include <iosfwd>
 #include <memory>
-#include <sstream>
 #include <mutex>
 
 #include "common.h"