Gate SOCK_DCCP behind its own ifdef (#15031)

AIX 7.1 has SOCK_CONN_DGRAM, but not SOCK_DCCP; the previous change
conflated the availability between the two definitions. Add an
additional #ifdef for this.

Author: NattyNarwhal

ext/sockets/sockets.stub.php

@@ -59,8 +59,10 @@
  * @cvalue SOCK_CONN_DGRAM
  */
 const SOCK_CONN_DGRAM = UNKNOWN;
+#endif
+#ifdef SOCK_DCCP
 /**
- * is an alias of SOCK_CONN_DGRAM
+ * is an alias of SOCK_CONN_DGRAM on some platforms
  * @var int
  * @cvalue SOCK_DCCP
  */