Replace unversioned GPL boilerplate notice with corresponding SPDX
license identifier, which is GPL 1.0+.

Cc: Donald Becker <becker@scyld.com>
Cc: Richard Hirst <richard@sleepie.demon.co.uk>
Cc: Sam Creasey <sammy@sammy.net>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>