Fix line spacing in `rndc secroots`

Fix line spacing in `rndc secroots`

Before this change, there was a missing blank line between the
negative trust anchors for one view, and the heading line for the next
view. This is because dns_ntatable_totext() omits the last newline.
There is an example of the incorrect output below; the fixed output
has a blank line before "Start view auth".

secure roots as of 21-Oct-2019 12:03:23.500:

 Start view rec
   Secure roots:

./RSASHA256/20326 ; managed

   Negative trust anchors:

example.com: expiry 21-Oct-2019 13:03:15.000
 Start view auth
   Secure roots:

./RSASHA256/20326 ; managed

   Negative trust anchors:

example.com: expiry 21-Oct-2019 13:03:07.000

(cherry picked from commit 5b600c2cd806515091a6ee3f818b700fe10147e6)