event/external: add missing include file

(cherry picked from commit open-mpi/ompi@4f8c0c5)

Author: ggouaillardet

opal/mca/event/external/external.h

@@ -2,6 +2,8 @@
  * Copyright (c) 2011-2015 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2013      Los Alamos National Security, LLC.  All rights reserved.
  * Copyright (c) 2015      Intel, Inc. All rights reserved.
+ * Copyright (c) 2015      Research Organization for Information Science
+ *                         and Technology (RIST). All rights reserved.
  *
  * $COPYRIGHT$
  *
@@ -17,6 +19,8 @@
 #ifndef MCA_OPAL_EVENT_EXTERNAL_H
 #define MCA_OPAL_EVENT_EXTERNAL_H
 
+#include "opal_config.h"
+
 #include "event.h"
 #include "event2/event.h"
 #include "event2/thread.h"