iio: mxs-lradc: add missing include

This patch adds the missing include for mutex handling.
Signed-off-by: NStefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: NMarek Vasut <marex@denx.de>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/mxs-lradc.c

@@ -26,6 +26,7 @@
 #include <linux/list.h>
 #include <linux/io.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/platform_device.h>
 #include <linux/spinlock.h>
 #include <linux/wait.h>