openvpn: let all openvpn variants provide a virtual openvpn package

Add PROVIDES:=openvpn to the default recipe in order to let all build variants
provide a virtual openvpn package.

The advantage of this approach is that downstream packages can depend on just
"openvpn" without having to require a specific flavor.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>