curlhttpsrc: compiler error on xenial

CURLINFO_CONTENT_LENGTH_DOWNLOAD was replaced with CURLINFO_CONTENT_LENGTH_DOWNLOAD_T in commit e9c68347, which results in compiler error below on xenial:

00:35:38,266 INFO - ../ext/curl/gstcurlhttpsrc.c: In function 'gst_curl_http_src_handle_response': 00:35:38,266 INFO - ../ext/curl/gstcurlhttpsrc.c:1315:44: error: 'CURLINFO_CONTENT_LENGTH_DOWNLOAD_T' undeclared (first use in this function) 00:35:38,266 INFO - if (curl_easy_getinfo (src->curl_handle, CURLINFO_CONTENT_LENGTH_DOWNLOAD_T,