kunit: Fix kernel-doc for EXPORT_SYMBOL_IF_KUNIT

While kunit/visibility.h is today not included in any generated
kernel documentation, also likely due to the fact that none of the
existing comments are correctly recognized as kernel-doc, but once
we decide to add this header and fix the tool, there will be:

../include/kunit/visibility.h:61: warning: Function parameter or
struct member 'symbol' not described in 'EXPORT_SYMBOL_IF_KUNIT'
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Acked-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

include/kunit/visibility.h

@@ -22,6 +22,7 @@
      * EXPORTED_FOR_KUNIT_TESTING namespace only if CONFIG_KUNIT is
      * enabled. Must use MODULE_IMPORT_NS(EXPORTED_FOR_KUNIT_TESTING)
      * in test file in order to use symbols.
+     * @symbol: the symbol identifier to export
      */
     #define EXPORT_SYMBOL_IF_KUNIT(symbol) EXPORT_SYMBOL_NS(symbol, \
 	    EXPORTED_FOR_KUNIT_TESTING)