Unbreak the build after the r1632907 merge.

* subversion/include/svn_error_codes.h
  (SVN_ERR_FS_INVALID_GENERATION): Manually copy definition from
                                   the revprop-caching-ng branch.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632909 13f79535-47bb-0310-9956-ffa450edef68