vmw_balloon: update copyright message

Removing the GPL wording and replace it with an SPDX tag. The immediate
trigger for doing it now is the need to remove the list of maintainers
from the source file, as the maintainer list changed.

Reviewed-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>