armbian-truncate-logs: Use journalctl --quiet (#3684)

By default, cron sends an email to the system admin if it finds anything in the stderr of the process. 
journalctl sends "diag output" to stderr (see https://github.com/systemd/systemd/issues/380).
The easiest way to prevent sporadic emails is to call journalctl with --quiet.

packages/bsp/common/usr/lib/armbian/armbian-truncate-logs

@@ -29,5 +29,5 @@ if [ $logusage -ge $treshold ]; then
     # remove
     /usr/bin/find /var/log -name '*.[0-9]' -or -name '*.gz' | xargs -r rm -f
     # vacuum systemd-journald
-    [ -d /var/log/journal ] && journalctl --vacuum-size=${JOURNAL_SIZE}
+    [ -d /var/log/journal ] && journalctl --quiet --vacuum-size=${JOURNAL_SIZE}
 fi