bit_spinlock: add required includes

Signed-off-by: Nick Piggin <npiggin@kernel.dk>
diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index 7113a32..e612575 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -1,6 +1,10 @@
 #ifndef __LINUX_BIT_SPINLOCK_H
 #define __LINUX_BIT_SPINLOCK_H
 
+#include <linux/kernel.h>
+#include <linux/preempt.h>
+#include <asm/atomic.h>
+
 /*
  *  bit-based spin_lock()
  *