Follow-up to r1618906: Resolve a deprecation warning.

* subversion/libsvn_wc/tree_conflicts.c
  (svn_wc__get_tree_conflict): Call non-deprecated
   svn_wc_conflict_description2_dup() function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618912 13f79535-47bb-0310-9956-ffa450edef68