Constify a few vtable pointers in the internal fs api.
Constify a few vtable pointers in the internal fs api.

* subversion/libsvn_fs/fs-loader.h
  (svn_fs_root_t,
   svn_fs_history_t,
   svn_fs_id_t): Mark vtable const.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1717575 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion