Make X3DDefs.h a private header


git-svn-id: https://root.cern.ch/svn/root/trunk@49223 27541ba8-7e3a-0410-8455-c3a389f83636
Fix https://sft.its.cern.ch/jira/browse/ROOT-5047


git-svn-id: https://root.cern.ch/svn/root/trunk@49225 27541ba8-7e3a-0410-8455-c3a389f83636