PEL: Const flattens for the SRC sub classes

Make the flatten functions const for the various classes
that represent the SRC substructures.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I08f4ae6de65e269c9f1b82315115969c8552c782
diff --git a/extensions/openpower-pels/callouts.cpp b/extensions/openpower-pels/callouts.cpp
index 925b383..17f5bdd 100644
--- a/extensions/openpower-pels/callouts.cpp
+++ b/extensions/openpower-pels/callouts.cpp
@@ -36,7 +36,7 @@
     }
 }
 
-void Callouts::flatten(Stream& pel)
+void Callouts::flatten(Stream& pel) const
 {
     pel << _subsectionID << _subsectionFlags << _subsectionWordLength;