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_header.hpp b/extensions/openpower-pels/section_header.hpp
index d87e4e5..139fc5d 100644
--- a/extensions/openpower-pels/section_header.hpp
+++ b/extensions/openpower-pels/section_header.hpp
@@ -98,7 +98,7 @@
  * @param[out] s - the stream
  * @param[in] header - the SectionHeader object
  */
-inline Stream& operator<<(Stream& s, SectionHeader& header)
+inline Stream& operator<<(Stream& s, const SectionHeader& header)
 {
     s << header.id << header.size << header.version << header.subType
       << header.componentID;