kconfig: use va_end to match corresponding va_start

Although on some systems va_end is a no-op, it is good practice
to use va_end, especially since the manual states:

"Each invocation of va_start() must be matched by a corresponding
invocation of va_end() in the same function."

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>