kconfig: fix missing fclose() on error paths

The file is not closed when ferror() fails.

Fixes: 00d674cb ("kconfig: refactor conf_write_dep()")
Fixes: 57ddd07c

 ("kconfig: refactor conf_write_autoconf()")
Reported-by: Ryan Cai <ycaibb@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>