hostip: Fix for builds that disable all asynchronous DNS

- Define Curl_resolver_error function only when USE_CURL_ASYNC.

Prior to this change building curl without an asynchronous resolver
backend (c-ares or threaded) and without DoH (DNS-over-HTTPS, which is
also asynchronous but independent of resolver backend) would cause a
build error since Curl_resolver_error is called by and evaluates
variables only available in asynchronous builds.

Reported-by: Benbuck Nason

Fixes https://github.com/curl/curl/issues/6831
Closes https://github.com/curl/curl/pull/6832

lib/hostip.c

@@ -1130,6 +1130,7 @@ CURLcode Curl_once_resolved(struct Curl_easy *data, bool *protocol_done)
  * resolve error
  */
 
+#ifdef USE_CURL_ASYNC
 CURLcode Curl_resolver_error(struct Curl_easy *data)
 {
   const char *host_or_proxy;
@@ -1153,3 +1154,4 @@ CURLcode Curl_resolver_error(struct Curl_easy *data)
 
   return result;
 }
+#endif /* USE_CURL_ASYNC */