add etc/security/limits.conf to exclude in mtree (#855) (#856)

(cherry picked from commit 9180867)

Co-authored-by: Caleb St. John <[email protected]>

Author: bugclerk

scale_build/image/mtree.py

@@ -39,6 +39,7 @@
     'etc/proftpd/proftpd.conf',
     'etc/proftpd/tls.conf',
     'etc/smartd.conf',
+    'etc/security/limits.conf',
     'etc/snmp/snmpd.conf',
     'etc/ssh/sshd_config',
     'etc/subuid',