PEL: Const flattens for the SRC sub classes

Make the flatten functions const for the various classes
that represent the SRC substructures.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I08f4ae6de65e269c9f1b82315115969c8552c782
diff --git a/extensions/openpower-pels/callout.hpp b/extensions/openpower-pels/callout.hpp
index ea28b7b..76eb0f9 100644
--- a/extensions/openpower-pels/callout.hpp
+++ b/extensions/openpower-pels/callout.hpp
@@ -62,7 +62,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    void flatten(Stream& pel);
+    void flatten(Stream& pel) const;
 
     /**
      * @brief Returns the priority field of a callout