explicitly load kernel-netlink plugin in UML scenarios

testing/tests/ikev2/host2host-cert/hosts/sun/etc/strongswan.conf

@@ -1,5 +1,5 @@
 # /etc/strongswan.conf - strongSwan configuration file
 
 charon {
-  load = curl aes des sha1 sha2 md5 gmp random x509 pubkey hmac xcbc stroke
+  load = curl aes des sha1 sha2 md5 gmp random x509 pubkey hmac xcbc stroke kernel-netlink
 }