iio: gts-helper: fix error headers

Both industrialio-gts-helper.c and iio-gts-helper.h use error codes
defined in <asm-generic/errno-base.h> (usually included via
<linux/errno.h>), but only industrialio-gts-helper.c includes it. On the
other hand, that file also uses error macros like IS_ERR() and
PTR_ERR(), which are included in <linux/err.h>.

Add the right includes to provide the used elements from the error
headers:

- <linux/errno.h> in iio-gts-helper.h

- <linux/err.h> in industrialio-gts-helper.c, which in turn includes
  <asm/errno.h>, which only includes <asm-generic/errno-base.h>

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>

drivers/iio/industrialio-gts-helper.c

@@ -5,7 +5,7 @@
  */
 
 #include <linux/device.h>
-#include <linux/errno.h>
+#include <linux/err.h>
 #include <linux/export.h>
 #include <linux/minmax.h>
 #include <linux/module.h>

include/linux/iio/iio-gts-helper.h

@@ -7,6 +7,7 @@
 #ifndef __IIO_GTS_HELPER__
 #define __IIO_GTS_HELPER__
 
+#include <linux/errno.h>
 #include <linux/types.h>
 
 struct device;