util: add missing FSF copyright statement

util: add missing FSF copyright statement

We previous added code for passing FDs which was explicitly derived from
gnulib's passfd code:

  commit 17460825f3c78e1635f2beb0165c5a19e3b09f7d
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri Jan 17 11:57:17 2020 +0000

    src: implement APIs for passing FDs over UNIX sockets

    This is a simplified variant of gnulib's passfd module
    without the portability code that we do not require.

while the license was unchanged, we mistakenly failed to copy the FSF
copyright header which is required by the license terms.

Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>