lib/fetcher: Add version to USER_AGENT string

This came up in allowing Fedora infrastructure to work around a libcurl bug with
HTTP2: https://pagure.io/atomic-wg/issue/405

Closes: https://github.com/ostreedev/ostree/issues/1405

Closes: #1406
Approved by: jlebon

src/libostree/ostree-fetcher-curl.c

@@ -714,7 +714,7 @@ initiate_next_curl_request (FetcherRequest *req,
     curl_easy_setopt (req->easy, CURLOPT_URL, uri);
   }
 
-  curl_easy_setopt (req->easy, CURLOPT_USERAGENT, "ostree ");
+  curl_easy_setopt (req->easy, CURLOPT_USERAGENT, OSTREE_FETCHER_USERAGENT_STRING);
   if (self->extra_headers)
     curl_easy_setopt (req->easy, CURLOPT_HTTPHEADER, self->extra_headers);