make Data_Make_Struct a statement to make it C99 compatible

include/mruby/data.h

@@ -28,11 +28,11 @@ struct RData *mrb_data_object_alloc(mrb_state *mrb, struct RClass* klass, void *
 #define Data_Wrap_Struct(mrb,klass,type,ptr)\
   mrb_data_object_alloc(mrb,klass,ptr,type)
 
-#define Data_Make_Struct(mrb,klass,strct,type,sval,data) (\
-  sval = mrb_malloc(mrb, sizeof(strct)),\
-  { static const strct zero = { 0 }; *sval = zero; },\
-  data = Data_Wrap_Struct(mrb,klass,type,sval)\
-)
+#define Data_Make_Struct(mrb,klass,strct,type,sval,data) do { \
+  sval = mrb_malloc(mrb, sizeof(strct));                     \
+  { static const strct zero = { 0 }; *sval = zero; };\
+  data = Data_Wrap_Struct(mrb,klass,type,sval);\
+} while (0)
 
 #define RDATA(obj)         ((struct RData *)((obj).value.p))
 #define DATA_PTR(d)        (RDATA(d)->data)