quiet clang warning by adding default move ctor (#1821)

include/pybind11/iostream.h

diff --git a/include/pybind11/iostream.h b/include/pybind11/iostream.h
index 72baef8..c43b7c9 100644
--- a/include/pybind11/iostream.h
+++ b/include/pybind11/iostream.h
@@ -64,6 +64,8 @@
         setp(d_buffer.get(), d_buffer.get() + buf_size - 1);
     }
 
+    pythonbuf(pythonbuf&&) = default;
+
     /// Sync before destroy
     ~pythonbuf() {
         sync();