kconfig: fix broken target update-po-config

Massimo Maiurana reported:
In the latest kernel "make update-po-config" fails because it tries
to open arch/Kconfig/Kconfig, since the ls command doesn't
distinguish between files and directories.

Cc: Massimo Maiurana <maiurana@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>