Make the tree-conflict WC APIs private to Subversion (but shared across libraries). Previously, two were public and the third was private to libsvn_wc. This is issue #3321.
* subversion/include/private/svn_wc_private.h (svn_wc__get_tree_conflict, svn_wc__add_tree_conflict): New, moved from svn_wc_get_tree_conflict and svn_wc_add_tree_conflict respectively in svn_wc.h. (svn_wc__del_tree_conflict): Moved from tree_conflicts.h.
* subversion/include/svn_wc.h (svn_wc_get_tree_conflict, svn_wc_add_tree_conflict): Rename and move to svn_wc_private.h.