Source code housekeeping trivia.

* subversion/include/private/svn_diff_tree.h
  (@file): Correct this Doxygen declaration to match the file name.
  (SVN_DIFF_TREE_H): Rename from SVN_DIFF_PROCESSOR_H to match
    the file name.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540752 13f79535-47bb-0310-9956-ffa450edef68