strongswan: write local_key to swanctl.conf

strongswan: write local_key to swanctl.conf

The 'local_key' UCI option was validated (checked for existence
under /etc/swanctl/private/) but never written to the generated
'swanctl.conf'. As a result, setting 'local_key' had no actual effect
on which private key was used for local authentication.

Add the corresponding "privkeys" line to the local{} section,
mirroring how local_cert is already written as "certs", so that
swanctl explicitly uses the configured private key.

Signed-off-by: Florian Eckert <redacted>