audit: remove duplicated include from audit.c

Remove duplicated include.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -60,7 +60,6 @@
 #include <linux/mutex.h>
 #include <linux/gfp.h>
 #include <linux/pid.h>
-#include <linux/slab.h>
 
 #include <linux/audit.h>