net/dsa/tag_ksz.c: remove superfluous headers

tag_ksz.c hasn't use any macro or function declared in linux/slab.h.
Thus, these files can be removed from tag_ksz.c safely without
affecting the compilation of the ./net/dsa module
Signed-off-by: Mianhan Liu <liumh1@shanghaitech.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_ksz.c

@@ -6,7 +6,6 @@
 
 #include <linux/etherdevice.h>
 #include <linux/list.h>
-#include <linux/slab.h>
 #include <net/dsa.h>
 #include "dsa_priv.h"