uhttpd: handle reload after uhttpd-mod-ubus installation using postinst

uhttpd: handle reload after uhttpd-mod-ubus installation using postinst

Use postinst script to reload service instead of uci-defaults hack. It's
possible thanks to recent base-files change that executes postinst after
uci-defaults.

This fixes support for uhttpd customizations. It's possible (again) to
adjust uhttpd config with custom uci-defaults before it gets started.

Cc: Hauke Mehrtens <hauke@hauke-m.de>
Fixes: d25d281fd668 ("uhttpd: Reload config after uhttpd-mod-ubus was added")
Ref: b799dd3c705d ("base-files: execute package's "postinst" after executing uci-defaults")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>