build fix

test/ares-test-internal.cc

@@ -583,7 +583,7 @@ TEST_F(LibraryTest, Striendstr) {
   const char *str = "plugh";
   EXPECT_NE(nullptr, ares_striendstr(str, str));
 }
-extern "C" ares_status_t ares__single_domain(ares_channel, const char*, char**);
+
 TEST_F(DefaultChannelTest, SingleDomain) {
   TempFile aliases("www www.google.com\n");
   EnvValue with_env("HOSTALIASES", aliases.filename());