luci-mod-system: add interface selection for NTPD

UCI option `interface` for sysntpd server was introduced in
OpenWrt commit 4da60500ebd2. NTP server binds to the specified interface,
or if unspecified, to all.

This patch adds selection widget to LuCI.

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>