thunderbolt: Fix build error in switch.c

Fixes the below error:
drivers/thunderbolt/switch.c:347:2: error: implicit declaration of function ‘kzalloc’ [-Werror=implicit-function-declaration]
drivers/thunderbolt/switch.c:381:2: error: implicit declaration of function ‘kcalloc’ [-Werror=implicit-function-declaration]
Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/switch.c

@@ -5,6 +5,7 @@
  */
 
 #include <linux/delay.h>
+#include <linux/slab.h>
 
 #include "tb.h"