luci-app-ddns: fix invalid markup

Fix invalid markup to be XHTML compliant in order to prevent exceptions
when opening DDNS pages with luci-theme-openwrt.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
Ref: https://forum.openwrt.org/t/19-07-luci-app-ddns-broken/57596
Fixes: 34fa5122

 ("luci-app-ddns: convert to client side implementatio")
[reword commit message, drop unrelated ui.js changes]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>