Merge pull request #4490 from dearblue/explicit-cast

Need explicit cast for C++

include/mruby/data.h

@@ -41,7 +41,7 @@ MRB_API struct RData *mrb_data_object_alloc(mrb_state *mrb, struct RClass* klass
 
 #define Data_Make_Struct(mrb,klass,strct,type,sval,data_obj) do { \
   (data_obj) = Data_Wrap_Struct(mrb,klass,type,NULL);\
-  (sval) = mrb_malloc(mrb, sizeof(strct));                     \
+  (sval) = (strct *)mrb_malloc(mrb, sizeof(strct));                     \
   { static const strct zero = { 0 }; *(sval) = zero; };\
   (data_obj)->data = (sval);\
 } while (0)