staging: add module.h to various iio drivers

Since they are assuming it is there implicitly and will fail otherwise
with things like:

drivers/staging/iio/impedance-analyzer/ad5933.c:816: warning: type
 defaults to ‘int’ in declaration of ‘MODULE_AUTHOR’

drivers/staging/iio/adc/ad7280a.c:990: warning: type defaults to ‘int’
 in declaration of ‘MODULE_AUTHOR’
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/adc/ad7280a.c

@@ -14,6 +14,7 @@
 #include <linux/err.h>
 #include <linux/delay.h>
 #include <linux/interrupt.h>
+#include <linux/module.h>
 
 #include "../iio.h"
 #include "../sysfs.h"

drivers/staging/iio/impedance-analyzer/ad5933.c

@@ -16,6 +16,7 @@
 #include <linux/types.h>
 #include <linux/err.h>
 #include <linux/delay.h>
+#include <linux/module.h>
 #include <asm/div64.h>
 
 #include "../iio.h"