Update struct initializer to work with relatively older C++.

src/class.c

@@ -1698,8 +1698,10 @@ mrb_define_module_function(mrb_state *mrb, struct RClass *c, const char *name, m
 static void
 mc_clear(mrb_state *mrb)
 {
+  static const struct mrb_cache_entry ce_zero ={0};
+
   for (int i=0; i<MRB_METHOD_CACHE_SIZE; i++) {
-    mrb->cache[i] = (struct mrb_cache_entry){0};
+    mrb->cache[i] = ce_zero;
   }
 }