Our ancient <arpa/nameser.h> lacks ns_*

Index: src/core/lib/event_engine/ares_resolver.cc
--- src/core/lib/event_engine/ares_resolver.cc.orig
+++ src/core/lib/event_engine/ares_resolver.cc
@@ -38,6 +38,7 @@
 
 #if ARES_VERSION >= 0x011200
 // c-ares 1.18.0 or later starts to provide ares_nameser.h as a public header.
+#undef CARES_HAVE_ARPA_NAMESER_H
 #include <ares_nameser.h>
 #else
 #include "src/core/lib/event_engine/nameser.h"  // IWYU pragma: keep
