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>

include/linux/export.h

@@ -82,7 +82,7 @@ extern struct module __this_module;
 #include <generated/autoksyms.h>
 
 #define __EXPORT_SYMBOL(sym, sec)				\
-	__cond_export_sym(sym, sec, config_enabled(__KSYM_##sym))
+	__cond_export_sym(sym, sec, __is_defined(__KSYM_##sym))
 #define __cond_export_sym(sym, sec, conf)			\
 	___cond_export_sym(sym, sec, conf)
 #define ___cond_export_sym(sym, sec, enabled)			\