build: luci.mk: Fix warning in Kconfig for Dutch language

When the Dutch translation was added there was no name for the language
added, this created such a Kconfig entry:

     config LUCI_LANG_nl
	   tristate " (nl)"

This entry created the following warning message when doing make
oldconfig in OpenWrt:
tmp/.config-package.in:24525:warning: leading whitespace ignored

This is fixed here by adding a name to the language.

Fixes: 1e301ca2

 ("Translated using Weblate (Dutch)")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>