ext/curl: Update stub CURLOPT_ENCODING @cvalue to CURLOPT_ACCEPT_ENCODING

In Curl 7.21.6, the `CURLOPT_ENCODING` constant was renamed to `CURLOPT_ACCEPT_ENCODING`.

`CURLOPT_ENCODING` is deprecated from Curl 7.21.6. It still provides
`CURLOPT_ENCODING` from `CURLOPT_ACCEPT_ENCODING` for backward compatibility.

However, now that PHP requires Curl 7.61, we can safely use the new `CURLOPT_ACCEPT_ENCODING`
constant.

Author: Ayesh

ext/curl/curl.stub.php

@@ -84,7 +84,8 @@
 const CURLOPT_EGDSOCKET = UNKNOWN;
 /**
  * @var int
- * @cvalue CURLOPT_ENCODING
+ * @cvalue CURLOPT_ACCEPT_ENCODING
+ * @alias CURLOPT_ACCEPT_ENCODING
  */
 const CURLOPT_ENCODING = UNKNOWN;
 /**