codegen: Update generateDS.py version to 2.12a

This update is required in order to make sure that empty XML elements
aren't lost during conversions.

Change-Id: I1fc08804b71aa316262c65ac5a8b6659db467251
Related: https://bugzilla.redhat.com/1091688
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>