openvpn-openssl: Compile management interface by default

[kosli]

resolves #21534

Compile tested: OpenWRT 22.03.5, OpenVPN 2.5.7, x86_64 openvpn-openssl & openvpn-mbedtls. Whereas opevpn-wolfssl failed. Run tested: OpenWRT 22.03.5, OpenVPN 2.5.7, x86_64 all my existing OpenVPN server and client connections worked fine, especially the auth-user-pass-verify featured finally worked as expected. I have tested only the openvpn-openssl package.

This patch is only for enabling it on the openvpn-openssl package.