lib/raid6: avoid __attribute_const__ redefinition

This is defined in glibc's sys/cdefs.h on my system with the same
definition as the raid6test fallback definition.  Add a #ifndef check to
avoid a compiler warning about redefining it.

Signed-off-by: Daniel Verkamp <dverkamp@chromium.org>
Signed-off-by: Shaohua Li <shli@fb.com>