openvpn: bump version to 2.4.0

openvpn: bump version to 2.4.0

--enable-password-save option has been removed
(https://github.com/OpenVPN/openvpn/commit/9ffd00e7541d83571b9eec087c6b3545ff68441f). Since
it now defaults to "enabled" in the upstream package, there is no
point in adding Config.in.legacy support for it: Config.in.legacy
logic only kicks in when the option is enabled, but the upstream
package precisely preserve the compatibility with this situation.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>