Add file to add snmp-mibs-downloader from non-free

sources/extra_files/app/preferences.d/snmp-mibs-downloader_nonfree

@@ -0,0 +1,4 @@
+Explanation: Enable package snmp-mibs-downloader from non-free tree (Zabbix app install with non-free option enable)
+Package: snmp-mibs-downloader
+Pin: release o=Debian,a=stable,l=Debian,c=non-free
+Pin-Priority: 600