luci-lib-nixio: Fix add_luci_conffiles adding duplicate files (#6568)

* luci-lib-nixio: Fix add_luci_conffiles adding duplicate files

add_luci_conffiles does not check whether the file already exists
when adding the file, which may result in redundant backups in the
sysupgrade backup.

Signed-off-by: Xiang W <wxjstz@126.com>