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>