mosquitto: add missing 'persistence' section in config

Section 'Persistence' in 'luci-app-mosquitto' is unusable without 'persistence'
section in config file.

Signed-off-by: Ptilopsis Leucotis <PtilopsisLeucotis@yandex.com>

net/mosquitto/files/etc/config/mosquitto

@@ -2,3 +2,5 @@ config owrt owrt
     option use_uci 0
 
 config mosquitto mosquitto
+
+config persistence persistence