can: slcan: Add missing linux/sched.h include.

drivers/net/can/slcan.c: In function 'slcan_open':
drivers/net/can/slcan.c:568: error: dereferencing pointer to incomplete type

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/can/slcan.c

@@ -53,6 +53,7 @@
 #include <linux/rtnetlink.h>
 #include <linux/if_arp.h>
 #include <linux/if_ether.h>
+#include <linux/sched.h>
 #include <linux/delay.h>
 #include <linux/init.h>
 #include <linux/can.h>