iio: st_accel: Fix unused variable warning

drivers/iio/accel/st_accel_core.c:1005:44: warning:
 mount_matrix_ext_info defined but not used [-Wunused-const-variable=]

Using stub helper while CONFIG_ACPI is disabled to fix it.
Suggested-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/st_accel_core.c

@@ -992,6 +992,7 @@ static const struct iio_trigger_ops st_accel_trigger_ops = {
 #define ST_ACCEL_TRIGGER_OPS NULL
 #endif
 
+#ifdef CONFIG_ACPI
 static const struct iio_mount_matrix *
 get_mount_matrix(const struct iio_dev *indio_dev,
 		 const struct iio_chan_spec *chan)
@@ -1012,7 +1013,6 @@ static const struct iio_chan_spec_ext_info mount_matrix_ext_info[] = {
 static int apply_acpi_orientation(struct iio_dev *indio_dev,
 				  struct iio_chan_spec *channels)
 {
-#ifdef CONFIG_ACPI
 	struct st_sensor_data *adata = iio_priv(indio_dev);
 	struct acpi_buffer buffer = {ACPI_ALLOCATE_BUFFER, NULL};
 	struct acpi_device *adev;
@@ -1140,10 +1140,14 @@ static int apply_acpi_orientation(struct iio_dev *indio_dev,
 out:
 	kfree(buffer.pointer);
 	return ret;
+}
 #else /* !CONFIG_ACPI */
+static int apply_acpi_orientation(struct iio_dev *indio_dev,
+				  struct iio_chan_spec *channels)
+{
 	return 0;
-#endif
 }
+#endif
 
 /*
  * st_accel_get_settings() - get sensor settings from device name