dco_linux: update license for ovpn_dco_linux.h

dco_linux: update license for ovpn_dco_linux.h

The linux userspace API header has acquired the MIT license (check the
ovpn-dco repository for the related change), therefore we simply bring
this change in our local copy to ensure compliancy.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230125095321.23063-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26077.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>