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/section.hpp b/extensions/openpower-pels/section.hpp
index d0f88e1..9fe3893 100644
--- a/extensions/openpower-pels/section.hpp
+++ b/extensions/openpower-pels/section.hpp
@@ -26,7 +26,7 @@
     /**
      * @brief Returns a reference to the SectionHeader
      */
-    SectionHeader& header()
+    const SectionHeader& header() const
     {
         return _header;
     }
@@ -44,7 +44,7 @@
      *
      * @param[in] stream - The stream to write to
      */
-    virtual void flatten(Stream& stream) = 0;
+    virtual void flatten(Stream& stream) const = 0;
 
   protected:
     /**