mpool/hugepage: add missing header

Signed-off-by: Nathan Hjelm <[email protected]>

Author: hjelmn

opal/mca/mpool/hugepage/mpool_hugepage.h

@@ -11,7 +11,7 @@
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
  * Copyright (c) 2006      Voltaire. All rights reserved.
- * Copyright (c) 2011-2015 Los Alamos National Security, LLC. All rights
+ * Copyright (c) 2011-2018 Los Alamos National Security, LLC. All rights
  *                         reserved.
  *
  * $COPYRIGHT$
@@ -29,6 +29,7 @@
 #include "opal_config.h"
 #include "opal/class/opal_list.h"
 #include "opal/class/opal_free_list.h"
+#include "opal/class/opal_rb_tree.h"
 #include "opal/mca/event/event.h"
 #include "opal/mca/mpool/mpool.h"
 #include "opal/util/proc.h"