Esnure \+ is not \outer during expl3 loading

This is the one place in expl3 that \+ is used in a place where this is
a potetial issue.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4699 de43f980-851b-0410-b2f7-c40aca1f87e0