Resolve another compiler warning.

* subversion/libsvn_ra_svn/client.c
  (find_tunnel_agent): Add non-const local variable ARGV to fix compiler 
   warning.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1626247 13f79535-47bb-0310-9956-ffa450edef68