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>