Silence gcc initializer warning.

src/vm.c

@@ -101,7 +101,7 @@ static inline void
 stack_clear(mrb_value *from, size_t count)
 {
 #ifndef MRB_NAN_BOXING
-  const mrb_value mrb_value_zero = { { 0 } };
+  const mrb_value mrb_value_zero = { 0 };
 
   while (count-- > 0) {
     *from++ = mrb_value_zero;