luci-proto-wireguard: grant uci read access to system and ddns config

The configuration export requires access to /e/c/system and /e/c/ddns for
external hostname hints.

Fixes: #5838
Fixes: 9ba20645

 ("luci-proto-wireguard: rewrite protocol handler")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>