iio: st_lsm6dsx: Mark predefined constants 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 predefined constants with __maybe_unused to calm a compiler down:

In file included from drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_i2c.c:17:
.../st_lsm6dsx/st_lsm6dsx.h:399:28: warning: ‘st_lsm6dsx_available_scan_masks’ defined but not used [-Wunused-const-variable=]
  399 | static const unsigned long st_lsm6dsx_available_scan_masks[] = {0x7, 0x0};
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../st_lsm6dsx/st_lsm6dsx.h:392:36: warning: ‘st_lsm6dsx_event’ defined but not used [-Wunused-const-variable=]
  392 | static const struct iio_event_spec st_lsm6dsx_event = {
      |                                    ^~~~~~~~~~~~~~~~
...

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