core: don't use the binary control for 2.6.34 and earlier

While the binary control file in sysfs is a neat feature, the
interface to binary attributes was different before 2.6.35. This
allows compiling with no warnings (and no crashes if the feature is
used) by just disabling the attribute at compile time.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Federico Vaga <federico.vaga@gmail.com>