dco-win: use run-time dynamic linking for GetOverlappedResultEx

dco-win: use run-time dynamic linking for GetOverlappedResultEx

This function is available starting from Windows 8. Calling it
"as is" causes startup error on Windows 7.

dco-win driver available on Windows 10 20H1 and newer. On older
systems installer will not show nor install the driver and dco-win code
won't be reached. It is safe to load GetOverlappedResultEx in runtime
and exit in case of error.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220820084719.243-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25038.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>