dnsmasq: add ubus acl to allow calls to hotplug.tftp object

dnsmasq may call hotplug.dhcp, hotplug.neigh and hotplug.tftp.
Only the first two callees were listed in the ACL, so add missing
hotplug.tftp.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

package/network/services/dnsmasq/files/dnsmasq_acl.json

@@ -7,6 +7,9 @@
 		},
 		"hotplug.neigh": {
 			"methods": [ "call" ]
+		},
+		"hotplug.tftp": {
+			"methods": [ "call" ]
 		}
 	}
 }