Follow-up to r1459815, moving a private function that depends on a
private structure into a private header so one of our build nanny
scripts doesn't complain.

* subversion/include/svn_ra_svn.h
  (svn_ra_svn__set_shim_callbacks): Move this from here ...

* subversion/include/private/svn_ra_svn_private.h
  ... to here, a(nother) new private header.

* subversion/libsvn_ra_svn/ra_svn.h,
* subversion/svnserve/serve.c
  Fun with #include.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1459844 13f79535-47bb-0310-9956-ffa450edef68