PEL: Make PEL::flatten() const

This includes making the flatten() method const in the PEL section base
class and in all of its derived classes.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I6be787962c6d7dfa01bdced2f9024564e6ac1b08
diff --git a/extensions/openpower-pels/src.hpp b/extensions/openpower-pels/src.hpp
index ab29c0b..e6f7029 100644
--- a/extensions/openpower-pels/src.hpp
+++ b/extensions/openpower-pels/src.hpp
@@ -82,7 +82,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    void flatten(Stream& stream) override;
+    void flatten(Stream& stream) const override;
 
     /**
      * @brief Returns the SRC version, which is a different field