Don't know. Have you reviewed the new documentation to see if it is mentioned in a "what's new" section.
Where were the header files installed in the previous release?
yes. file exists in tuxedo11gR3client/include folder that was installed
My guess is a bug in the new installer left out the installation of that file. I would suggest opening a support request at My Oracle Support. In the mean time you should be able to safely copy the old version over for now. As far as I know, there have been no changes to the file.
Oracle Tuxedo Chief Architect