staging: Add export.h for THIS_MODULE/EXPORT_SYMBOL to drivers/staging users.

Lots of drivers have in the past expected the presence of basic things
like THIS_MODULE and EXPORT_SYMBOL.  With the header cleanup, they wont
have these.  Call out the include explicitly.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/drivers/staging/iio/imu/adis16400_ring.c b/drivers/staging/iio/imu/adis16400_ring.c
index c368245..fd886bf 100644
--- a/drivers/staging/iio/imu/adis16400_ring.c
+++ b/drivers/staging/iio/imu/adis16400_ring.c
@@ -4,6 +4,7 @@
 #include <linux/spi/spi.h>
 #include <linux/slab.h>
 #include <linux/bitops.h>
+#include <linux/export.h>
 
 #include "../iio.h"
 #include "../ring_sw.h"