iio: gyro: st_gyro: Mark gyro_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 gyro_pdata with __maybe_unused to calm a compiler down:

In file included from drivers/iio/gyro/st_gyro_i2c.c:18:
drivers/iio/gyro/st_gyro.h:31:46: warning: ‘gyro_pdata’ defined but not used [-Wunused-const-variable=]
   31 | static const struct st_sensors_platform_data gyro_pdata = {
      |                                              ^~~~~~~~~~
...

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>