tile: add #include to unbreak build after generic init_task conversion

Some code was moved from init_task.c to setup.c but the appropriate
header needed to be moved as well.
Signed-off-by: NChris Metcalf <cmetcalf@tilera.com>

arch/tile/kernel/setup.c

@@ -29,6 +29,7 @@
 #include <linux/smp.h>
 #include <linux/timex.h>
 #include <linux/hugetlb.h>
+#include <linux/start_kernel.h>
 #include <asm/setup.h>
 #include <asm/sections.h>
 #include <asm/cacheflush.h>