Including <linux/bit_spinlock.h> into an empty TU will result in the
compiler complaining:

./include/linux/bit_spinlock.h:34:4: error: call to undeclared function 'cpu_relax'; <...>
   34 |                         cpu_relax();
      |                         ^
1 error generated.

Include <asm/processor.h> to allow including bit_spinlock.h where
<asm/processor.h> is not otherwise included.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/bit_spinlock.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/bit_spinlock.h b/include/linux/bit_spinlock.h
index c0989b5b0407..59e345f74b0e 100644
--- a/include/linux/bit_spinlock.h
+++ b/include/linux/bit_spinlock.h
@@ -7,6 +7,8 @@
 #include <linux/atomic.h>
 #include <linux/bug.h>
 
+#include <asm/processor.h>  /* for cpu_relax() */
+
 /*
  *  bit-based spin_lock()
  *
-- 
2.51.0.384.g4c02a37b29-goog