Add "AND CONTRIBUTORS" to bupsplit.h and bupsplit.c licenses

Add "AND CONTRIBUTORS" to bupsplit.h and bupsplit.c licenses

From https://groups.google.com/d/msg/bup-list/8lcxXuXilTE/UyOe7VGuCQAJ

  From: Avery Pennarun <apenwarr@gmail.com>
  Date: Sun, 18 Mar 2018 18:35:15 -0400
  Message-ID: <CAHqTa-1ghU6+Y0Y2pBOjbS=7CWKMytPvj-c1Z0aE3=PqpPi1OA@mail.gmail.com>
  Subject: Re: bupsplit.c copyright and patching
  To: Rob Browning <rlb@defaultvalue.org>
  Cc: Robert Evans <evansr@google.com>, bup-list <bup-list@googlegroups.com>
    ...

  > So for the record, it sounds like you approve of changing the relevant
  > bupsplit.h and bupsplit.c phrase to this?
  >
  >   THIS SOFTWARE IS PROVIDED BY AVERY PENNARUN AND CONTRIBUTORS``AS IS'' AND ANY

  Yes, that's fine.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>