add cstdio header, seems to be missing when compiling with clang (thanks to Rene Milk)

[[Imported from SVN: r6208]]

dune/common/collectivecommunication.hh

@@ -213,7 +213,7 @@ namespace Dune
     void allreduce(Type* in, Type* out, int len) const
     {
       std::copy(in, in+len, out);
-      return 0;
+      return;
     }
 
   };