supress cast warning on C++.

include/mruby/data.h

@@ -40,7 +40,7 @@ struct RData *mrb_data_object_alloc(mrb_state *mrb, struct RClass* klass, void *
 void *mrb_get_datatype(mrb_state *mrb, mrb_value, const struct mrb_data_type*);
 void *mrb_check_datatype(mrb_state *mrb, mrb_value, const struct mrb_data_type*);
 #define Data_Get_Struct(mrb,obj,type,sval) do {\
-  sval = mrb_check_datatype(mrb, obj, type); \
+  *(void**)&sval = mrb_check_datatype(mrb, obj, type); \
 } while (0)
 
 #if defined(__cplusplus)