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> (cherry picked from commit 24ac5a2bf6d713c6878cf7be7d4e4516965c2884)