resolve: fix building without Unix sockets and `CURLDEBUG`

resolve: fix building without Unix sockets and `CURLDEBUG`

```
In file included from server_bundle.c:7:
../../../tests/server/resolve.c:110:5: error: unknown type name 'curl_socket_t'; did you mean 'curl_socklen_t'?
    curl_socket_t s = socket(PF_INET6, SOCK_DGRAM, 0);
    ^~~~~~~~~~~~~
    curl_socklen_t
../../../include/curl/system.h:392:38: note: 'curl_socklen_t' declared here
  typedef CURL_TYPEOF_CURL_SOCKLEN_T curl_socklen_t;
                                     ^
In file included from server_bundle.c:7:
../../../tests/server/resolve.c:111:13: error: use of undeclared identifier 'CURL_SOCKET_BAD'
    if(s == CURL_SOCKET_BAD)
            ^
```
Ref: https://github.com/curl/curl/actions/runs/13825438937/job/38679418428?pr=15000#step:14:47

Cherry-picked from #15000
Closes #16700