iio: Add missing include guards to headers

Add include guards to the IIO headers where they are missing. This avoids
compile errors due to redefined types if a file is included more than once.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/include/linux/iio/kfifo_buf.h b/include/linux/iio/kfifo_buf.h
index 014d5a1..25eeac7 100644
--- a/include/linux/iio/kfifo_buf.h
+++ b/include/linux/iio/kfifo_buf.h
@@ -1,3 +1,5 @@
+#ifndef __LINUX_IIO_KFIFO_BUF_H__
+#define __LINUX_IIO_KFIFO_BUF_H__
 
 #include <linux/kfifo.h>
 #include <linux/iio/iio.h>
@@ -6,3 +8,4 @@
 struct iio_buffer *iio_kfifo_allocate(struct iio_dev *indio_dev);
 void iio_kfifo_free(struct iio_buffer *r);
 
+#endif