gnu-maintenance: Add newline in warning messages.

* guix/gnu-maintenance.scm (download-tarball): Add newline in warning
  messages.

guix/gnu-maintenance.scm

@@ -356,9 +356,9 @@ (define* (download-tarball store project directory version
       (if ret
           tarball
           (begin
-            (warning (_ "signature verification failed for `~a'")
+            (warning (_ "signature verification failed for `~a'~%")
                      base)
-            (warning (_ "(could be because the public key is not in your keyring)"))
+            (warning (_ "(could be because the public key is not in your keyring)~%"))
             #f)))))
 
 (define (package-update store package)