eina_cxx: allow for the generation of function that return Eina_Promise in .eo files.

Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Differential Revision: https://phab.enlightenment.org/D7576

src/bindings/cxx/eina_cxx/eina_deleter.hh

@@ -23,6 +23,10 @@ struct malloc_deleter
   {
     // how to free binbuf?
   }
+  void operator()(Eina_Promise*) const
+  {
+    // workaround until we manually handle efl_loop_promise_new
+  }
 };
 
 template <typename T>