Make the default RB_AUGMENT() produce a 'do {} while (0)' instead

of nothing. This prevents the compiler from complaining about empty
if statements when compiled with higher WARN levels.

sys/sys/tree.h

@@ -332,7 +332,7 @@ struct {								\
 } while (/*CONSTCOND*/ 0)
 
 #ifndef RB_AUGMENT
-#define RB_AUGMENT(x)
+#define RB_AUGMENT(x)	do {} while (0)
 #endif
 
 #define RB_ROTATE_LEFT(head, elm, tmp, field) do {			\