Mark destructor of DebugStream as noexcept(false)

C++11 defaults destructors to noexcept.
Good catch, GCC 6!

dune/common/debugstream.hh

@@ -237,7 +237,8 @@ namespace Dune {
        thrown. Otherwise the child streams would certainly break on the
        next output
      */
-    ~DebugStream() {
+    ~DebugStream() noexcept(false)
+    {
       // untie
       if (_tied)
         tiedstate->_tied_streams--;