xmlrpc-c: update 1.25.25 -> 1.25.26

From http://xmlrpc-c.sourceforge.net/change_super_stable.html:

  Release 1.25.26, released September 29, 2013.

  Fix interpretation of dateTime.iso8601 variant with timezone
  information. As broken, it would add up to a second to the correct
  datetime or reject the message claiming that it has invalid syntax.
  Always broken (recognition of the timezone variant was introduced in
  Xmlrpc-c 1.20 (September 2009).