Fix a compiler warning

Fix a compiler warning

[1/16] Compiling C object libevdev.so.2.3.0.p/libevdev_libevdev.c.o
../libevdev/libevdev.c:665:40: warning: argument 2 of type ‘struct slot_change_state[dev->num_slots]’ declared as a variable length array [-Wvla-parameter]
  665 |               struct slot_change_state changes_out[dev->num_slots])
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
../libevdev/libevdev.c:47:52: note: previously declared as a pointer ‘struct slot_change_state *’
   47 |                          struct slot_change_state *changes_out);
      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>