Add missing include, exposed in "external libevent" work.

Refs trac:3694

This commit was SVN r29898.

The following Trac tickets were found above:
  Ticket 3694 --> https://svn.open-mpi.org/trac/ompi/ticket/3694

ompi/mca/btl/openib/btl_openib_fd.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008-2009 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2008-2013 Cisco Systems, Inc.  All rights reserved.
  * Copyright (c) 2009 Sandia National Laboratories. All rights reserved.
  *
  * $COPYRIGHT$
@@ -37,6 +37,7 @@
 #include "opal/mca/event/event.h"
 #include "opal/util/output.h"
 #include "opal/util/fd.h"
+#include "opal/threads/threads.h"
 
 #include "ompi/constants.h"