greybus: Fix build errors on older kernels.
Thanks to Marti for pointing out the code didn't build properly on 3.10.
Added kernel_ver.h to handle any api mis-matches between the code and
older kernel versions.
diff --git a/drivers/staging/greybus/sysfs.c b/drivers/staging/greybus/sysfs.c
index 2393a6a..c02ca55 100644
--- a/drivers/staging/greybus/sysfs.c
+++ b/drivers/staging/greybus/sysfs.c
@@ -18,6 +18,8 @@
#include "greybus.h"
+#include "kernel_ver.h"
+
/* Function fields */
#define greybus_function_attr(field) \
static ssize_t function_##field##_show(struct device *dev, \