ptr_ring: remove duplicated include from ptr_ring.h

Remove duplicated include.
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/ptr_ring.h

@@ -26,7 +26,6 @@
 #include <linux/cache.h>
 #include <linux/types.h>
 #include <linux/compiler.h>
-#include <linux/cache.h>
 #include <linux/slab.h>
 #include <asm/errno.h>
 #endif