MAINTAINERS: add Sabrina as official reviewer for ovpn

MAINTAINERS: add Sabrina as official reviewer for ovpn

Sabrina put quite some effort in reviewing the ovpn module
during its official submission to netdev.
For this reason she obtain extensive knowledge of the module
architecture and implementation.

Make her an official reviewer, so that I can be supported
in reviewing and acking new patches.

Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>