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/extended_user_header.hpp b/extensions/openpower-pels/extended_user_header.hpp
index e964bd4..e30d771 100644
--- a/extensions/openpower-pels/extended_user_header.hpp
+++ b/extensions/openpower-pels/extended_user_header.hpp
@@ -70,7 +70,7 @@
      */
     size_t flattenedSize()
     {
-        return Section::flattenedSize() + _mtms.flattenedSize() +
+        return Section::headerSize() + _mtms.flattenedSize() +
                _serverFWVersion.size() + _subsystemFWVersion.size() +
                sizeof(_reserved4B) + sizeof(_refTime) + sizeof(_reserved1B1) +
                sizeof(_reserved1B2) + sizeof(_reserved1B3) +