iio: accel: st_accel: Mark default_accel_pdata with __maybe_unused

Since we put static variable to a header file it's copied to each module
that includes the header. But not all of them are actually used it.

Mark default_accel_pdata with __maybe_unused to calm a compiler down:

In file included from drivers/iio/accel/st_accel_i2c.c:19:
drivers/iio/accel/st_accel.h:67:46: warning: ‘default_accel_pdata’ defined but not used [-Wunused-const-variable=]
   67 | static const struct st_sensors_platform_data default_accel_pdata = {
      |                                              ^~~~~~~~~~~~~~~~~~~
...
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/accel/st_accel.h

@@ -64,7 +64,7 @@ enum st_accel_type {
 * struct st_sensors_platform_data - default accel platform data
 * @drdy_int_pin: default accel DRDY is available on INT1 pin.
 */
-static const struct st_sensors_platform_data default_accel_pdata = {
+static __maybe_unused const struct st_sensors_platform_data default_accel_pdata = {
 	.drdy_int_pin = 1,
 };