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/src.cpp b/extensions/openpower-pels/src.cpp
index 43bf205..919c411 100644
--- a/extensions/openpower-pels/src.cpp
+++ b/extensions/openpower-pels/src.cpp
@@ -379,7 +379,7 @@
 
     _size = baseSRCSize;
     _size += _callouts ? _callouts->flattenedSize() : 0;
-    _header.size = Section::flattenedSize() + _size;
+    _header.size = Section::headerSize() + _size;
 
     _valid = true;
 }