can: softing_cs needs slab.h

From: Randy Dunlap <randy.dunlap@oracle.com>

softing_cs.c uses kzalloc & kfree, so it needs to include linux/slab.h.

drivers/net/can/softing/softing_cs.c:234: error: implicit declaration of function 'kfree'
drivers/net/can/softing/softing_cs.c:271: error: implicit declaration of function 'kzalloc'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>