ompi/debuggers: stomp a compiler warning in dlopen_test.c

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

ompi/debuggers/dlopen_test.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2009-2018 Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -12,6 +12,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "opal/runtime/opal.h"
 #include "opal/mca/dl/base/base.h"