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>

include/linux/raid/pq.h

@@ -53,7 +53,9 @@ extern const char raid6_empty_zero_page[PAGE_SIZE];
 
 #define __init
 #define __exit
-#define __attribute_const__ __attribute__((const))
+#ifndef __attribute_const__
+# define __attribute_const__ __attribute__((const))
+#endif
 #define noinline __attribute__((noinline))
 
 #define preempt_enable()