export.h: use __is_defined() to check if __KSYM_* is defined

Here the need is for a macro that checks whether the given symbol is
defined or not, which has nothing to do with config.

The new macro __is_defined() is a better fit for this case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.com>