PEL: Rename Section::flattenedSize

cppcheck was complaining about how flattenedSize() was in both the base
class Section and all of its derived classes even though it wasn't
virtual.

Change-Id: I5cce8c6534e696f30c2c6141a50665d11a13eff0
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/extensions/openpower-pels/user_data.hpp b/extensions/openpower-pels/user_data.hpp
index 142a502..22340db 100644
--- a/extensions/openpower-pels/user_data.hpp
+++ b/extensions/openpower-pels/user_data.hpp
@@ -67,7 +67,7 @@
      */
     size_t flattenedSize()
     {
-        return Section::flattenedSize() + _data.size();
+        return Section::headerSize() + _data.size();
     }
 
     /**