Fix r20917 -- some systems require <unistd.h> for !_SC_PAGESIZE.

This commit was SVN r20920.

The following SVN revision numbers were found above:
  r20917 --> open-mpi/ompi@d10393a925

opal/mca/maffinity/first_use/maffinity_first_use_module.c

@@ -9,6 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
+ * Copyright (c) 2009      Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -20,6 +21,8 @@
 
 #include <string.h>
 #include <stddef.h>
+#include <unistd.h>
+
 #include "opal/constants.h"
 #include "opal/mca/maffinity/maffinity.h"
 #include "opal/mca/maffinity/base/base.h"