Make svnauthz-validate symlink relative.

Improves relocatability of the install.

* Makefile.in:
  (INSTALL_EXTRA_TOOLS): remove explicit path on source of svnauthz-validate
    symlink.

Suggested by: danielsh


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428712 13f79535-47bb-0310-9956-ffa450edef68