Followup to r1436580: Add a const to silence a compiler warning.

* subversion/libsvn_client/patch.c
  (get_hunk_info): Constify a variable.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436622 13f79535-47bb-0310-9956-ffa450edef68