ntp: add hotplug class for ntp-events

Based on a patch by Luiz Angelo Daros de Luca

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43421 3c298f89-4303-0410-b956-a3cf2f4a3e73