btl/tcp: add missing #include <unistd.h>

Thanks Marco Atzeri for contributing the original patch

(cherry picked from commit open-mpi/ompi@5fa63f0)

opal/mca/btl/tcp/btl_tcp.h

@@ -11,7 +11,7 @@
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
  * Copyright (c) 2010-2011 Cisco Systems, Inc.  All rights reserved.
- * Copyright (c) 2014      Research Organization for Information Science
+ * Copyright (c) 2014-2015 Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2014-2015 Los Alamos National Security, LLC. All rights
  *                         reserved.
@@ -37,6 +37,9 @@
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
 #endif
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 
 /* Open MPI includes */
 #include "opal/mca/event/event.h"