vtysh: add missing rpki node when showing config

vtysh: add missing rpki node when showing config

Before:
```
frr version 8.2-dev
frr defaults traditional
hostname frr
rpki
 rpki polling_period 20
 rpki cache 192.0.2.1 8080 preference 1
service integrated-vtysh-config
!
```

After:
```
frr version 8.2-dev
frr defaults traditional
hostname frr
service integrated-vtysh-config
!
rpki
 rpki polling_period 20
 rpki cache 192.0.2.1 8080 preference 1
exit
!
```

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>