url: Fix compile error with --enable-werror

lib/url.c

@@ -3786,6 +3786,8 @@ static void free_fixed_hostname(struct hostname *host)
   free(host->encalloc); /* must be freed withidn_free() since this was
                            allocated by curl_win32_idn_to_ascii */
   host->encalloc = NULL;
+#else
+  (void)host;
 #endif
 }