plinth: Set RestartSec to 30 seconds

This is meant to help with an issue where the plinth service happens to fail to start, and fails 5 times repeatedly in a short time: #1644 (comment 226753)

However there is not always a failure to start. I have not yet been able to confirm that this actually solves the multiple restarts issue.

Signed-off-by: James Valleroy jvalleroy@mailbox.org