hostip: Fix for builds that disable all asynchronous DNS

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